There are two steps of the technique that demand attention.  First, when cutting and dampening the colle paper you must learn to allow for the expansion of the paper. You may "test" it for size before sprinkling with glue and trim as necessary.  Secondly, when applying the glue, shake it out of a container thinly and evenly over the colle paper.  YOU DON'T NEED MUCH.  It is best to sprinkle the colle paper with glue before the final placing upon the plate, laying the colle paper on a piece of newsprint to collect waste glue. If the paper is sprinkled while positioned on the plate, the edges of the plate will have to be wiped risking touching or moving the colle paper.  I have done hundreds and hundreds of colles using this method with very few failures.  I use a plastic container (about 6"x2"diam.) covered with a couple layers of tarlatan for the glue applicator.  If you use proper press pressure and do not over sprinkle you will not ever have a problem with glue "oozing out" around the edges and will have excellent bondage.  After confidence is obtained doing the single colle, the real fun begins!  You can expand the concept to include doing several colles within a single print.  I call the method "multi-colle".  Specifically, you 

can place papers on selected areas or images of the plate.  A colle paper cut to the plate size can then be used, or not used, before laying the final (backing) paper upon the plate. Perhaps the most exciting reason to use the colle process is not only can numerous colored papers be used, but you may create your own, or use various other printed material. I often use maps, dictionary pages, or text book pages, but be aware of the  quality and acidity of the papers you select. When staining papers for a custom color a fadeproof colored ink seems to work best (color won't bleed or run when re-soaked) .  Using these later items can also encourage production of unique prints, either as variable editions, or as monoprints.

Let me explain a few important aspects of doing multi-colles.  To be consistent if editioning, I make a template for each piece placed upon the plate.  I use tracing paper to transfer the area or image from the plate to a thin cardboard template, which will in turn be used to cut out the colle paper. To begin, I assemble the papers on the plate off the press bed, placing it on the press bed only when completed.  I normally use a colle paper over the full plate as well as several cut pieces  in selected areas on the plate.  The only way I have found to successfully do this is to first place the dampened and sprinkled full paper on the plate, place 2 or 3 small weights on one side to keep it  in position, and then carefully lift and fold it back exposing the plate again, allowing you to then place the other dampened and sprinkled cut pieces on the plate.  After the cut pieces are placed, fold the full paper slowly and carefully back over the plate.  (See Figure 1).  Place this "sandwich" 
of papers on the press bed, lay the backing paper (BFK or other) over the assemblage and run it through the press.  Dampening of all the paper is important because it allows the paper to lie flat on the plate as well as improve the  printing quality.  Positioning the cut pieces on the plate is easy.  I use the tip of an X-ACTO knife to move the cut pieces correctly and precisely into position.  Be slow and patient and go easy on the caffeine before doing the multi-colle!  The results are very gratifying.

Why do I use cut papers instead of multiple plates?  Frankly, there are some occasions that I feel the best way to achieve my goal is to use multiple plates.  The time it takes to print the plate and the size of the edition is an important factor when considering this method.   I rarely make an edition more than 15 so cutting the paper pieces is not too demanding.  In most cases I can achieve the same or better effect by using cut papers.   Papers may be used that are highly saturated with  color giving a slightly brighter look than printed color in some cases.  There is also a cost savings using paper rather than an additional plate(s).   Another concern that is important to me is the ability to keep clean edges on my prints.  It is extremely difficult to make a multi-plate etching without having some registration error show on the beveled edges.  Although a slight amount may be considered acceptable, I still prefer clean edges.

Lastly, I would like to share a point about ink, and inking the plate, that should be considered when doing certain colles. I would like to caution against using a heavy, "tacky" ink with colles (this I found out the hard way).  Under certain conditions with rich, deep etched plates, you can tear or pull apart the colle paper as you remove the print from the plate(the tackiness of the ink is too much for the colle paper).  And an even more common effect of a very tacky ink, is pulling a slight pocket or bubble into the print as you remove the print from the plate.  What happens is the ink-to-paper bond is more dominate than the paper-to-paper glue bond, pulling the colle paper away from the backing paper as you remove the print.  A typical, buttery ink works just fine.

I start with an 1/8" piece of bathroom masonite(factory spray-painted on one side), a blotter, next the print, then newsprint, another blotter, and finished with another 1/8" piece of masonite.  I would recommend not stacking  more than 5 or 6 prints in this manner.  About 35-50 lbs. of weights are evenly placed over the last masonite.  You can use almost anything for weights especially bricks, books, or pieces of metal.  I have found that it is very important to get the moisture out as soon as possible and the key is changing the newsprint EARLY and OFTEN,  totaling 5-6 times.  The first change is done after 15 minutes, the second after about 30 more minutes, the third after about an hour, and the remaining changes are about every 2-3 hours.  Yes, this is a lot of work, but the results are excellent!  Another reason to change the newsprint often is because it wrinkles quickly, and if left too long, will offset the wrinkle pattern onto the ink on the drying print.  After a couple hours the ink has generally set up enough and this is no longer a problem.  Each time I change the newsprint I will "turn over" the blotters as I re-stack the pile.  After the last change I allow the prints to continue to dry for about a day.  Needless to say there will always be variations to this schedule.

One of the best ways to dry prints is an in-expensive, simple, "forced air" drying system.  Essentially, it is very similar to the method above except the masonite is replaced with a heavy cardboard ( you can glue two thin pieces together using spray adhesives).  Using a trash bag as a flexible connection between an electric fan and the stack of prints, air is forced through the open spaces in the cardboard to pull out the moisture (see figure 2  below).  The prints will completely dry in hours although I allow them to dry overnight.  Even using this method, if my print(s) is "lush" with masses of ink, I will lightly blot it first and also change newsprint once or twice during the drying process.  Again, I limit the total number of prints that are stacked to about 4 or 5.  This actually works out well for me because that is about all the multi-colle prints I will do in a days printing using a 20"x16" or 24"x18" size plate.  I have seen a similar method as this used where plywood was placed between "packs" of prints and clamped tight using bar clamps and a larger fan used.  This allowed many more prints to be stacked at one time.

One final comment.  If, after your very best effort, you still have a wavy or wrinkled print, mist the back side with water for 20-30 minutes, sprinkle with wheat paste, and lay another piece of dampened print paper over it.  Dry the print again.  It will be flat!  This second piece of paper can be removed if necessary but I do not recommend it.  If you ever do add another sheet of paper like this, I suggest you note it on the back of the print using a pencil .

I welcome any comments you may have or possible questions.