I should take the easy way out and tell all to follow the manufacturers recommendations, I have and they work.
These methods have worked for me for years now, other methods might work as well.
1. Cut:
When cutting the decals from the sheet I use a pair of small dressmakers scissors which are very sharp. Leave about a 1/32" around the decal when cutting, this edge tends to settle down better than an edge cut right on the paint. I cut out all the decals I'm going to need on the model and keep them in a small box to prevent loss.2. Soak:
I use ordinary tap water from my well in a small cereal bowl to soak the decals. If your water causes spots etc, use bottled water from the grocery. I place the decals on the water and let the backing sheet get wet through. Back in the day when decal film was thicker, you could float the decal off the backing sheet. With our modern thin film decals, it is wise to leave the decal on the backing and remove both together. I then remove them together from the water and place them on board which is covered with Formica. I use a pair of tweezers to pick them up by an edge. Let them set till they are loose or ready to slide.3. Slide:
Using a small brush I put water mixed with solva-set on the area where the decal is to be placed. Using a diluted mixture of solva-set allows a bit more time to relocate the decal before it tends to soften and become un-moveable. Then slide the decal into place by holding one edge and pulling the backing sheet from under. Try to get the decal in its final place without much movement. When the decal is where it should be let it partially dry.4. Set:
Apply some setting solution with a small brush all around the edges and over the decal. Use a light touch here, don't scrub the decal with the brush. I use a # 0 sable hair brush for this step. I use Walthers "Solvaset" for all brands of decals. DON'T try to relocate the decal after application of "Solvaset', if it isn't where it should be, you will probably need another decal. I let this decal air dry and sometimes need a second coat of "Solvaset". Usually on long and wide decals such as the stripes on the Seaboard or Southern units. Let the decals dry thoroughly, I wait overnight, and with the same small brush use tap water and flood the area. Use a good brand of paper towell or a soft cloth and blot the excess water and stains. Again use a light touch. I sometimes use an old cotton "tee" shirt dampened and wipe lightly the stains.5. Protect:
Final protection is provided by what ever final finish you apply to the model. This can be done now or after applying any weathering. Some use Testors "Dullcote", Testors "Glosscote", clear lacquer for gloss or add talcum powder for a dull finish. My personal preference for a gloss finish is Johnson "Future Floor Finish". This comes in a 27 ounce bottle, available at most grocery stores or Wal-Mart. Spray on full strength with an airbrush, clean up same as for any acrylic. The Future can be sprayed over the entire model including window glazing etc. Only makes the glazing more shiny and clear. For a semi-gloss a good mix is half and half "Testors Glosscote and Dullcote". For a dull or flat finish I use Testors "Dullcote". Neither Glosscote or Dullcote should be sprayed over(onto) the windows, these must be masked off to prevent glazing. These also come in a spray can so no airbrush cleanup.Anyway when the decals are on it is time to do Part 6 WEATHERING to the model.