I have found this process to work very well. Usually my decals look just like they were painted on.
Clean all surfaces with mild detergent and warm water. Handle them with vinyl gloves (they look like surgical gloves, but aren't sterile--you can get them in 50-packs at your drug store). You want to remove any mould release agents as well as the skin oils deposited when you built the model. Blot with a lint free cloth or air dry. The model needn't be absolutely dry, after all, decaling is a water-based process, but you don't want any beads of water on the model either. If your water is particularly hard and leaves calcium deposits on the model you should make a final rise with distilled water.
Trim all decals from the stock as close to the decal as possible. Dip the decals in water for the specified time. Apply a decal solvent, Solvaset is good, to the model surface where you want to place the decal. Slide the decal from the backing paper to the surface and quickly position it. Within a short time, make that a very short time, the decal will soften to the point that further attempts to move it will stretch and distort it beyond usefulness. You will have no recourse but to remove the decal and use a replacement.
After a few minutes, you will see the decal appear to bubble up and look, in a word, awful. Resist all urges to pat the decal down, or otherwise touch it. Be patient. After a bit, it will snuggle down quite well. There may be the odd bubble or more in the middle of the decal, but let the decal dry thoroughly. The next day, puncture the bubbles with a sharp pin or the tip of a brand new No. 11 X-Acto blade. Apply more Solvaset to the decal, with special emphases on the points of puncture.
You should be quite satisfied with the final appearance of the decals. I know I am. If you find Solvaset works too quickly, try Micro-Sol and Micro-Set. They seem to me, at least, to be slower acting. A compromise that I have found in cases where I need to make repeated small adjustments, like with long thin decals (stripes, long names in small type size), is to use slower acting Micro-Sol below the decal, then when it is dry, Solvaset on top. In some cases, I have applied the decal without any solvent underneath (i.e., not applied to the model first). After the decal was positioned the way I liked, I used a brush paint a line of Solvaset along the edge of the decal so that capillary action pulled the solvent under the decal. Then I carefully painted the upper surface with the solvent and put it down until it was dry. The trick is to get the overcoat on before the undercoat has had time to soften the decal. Remember, once the decal softens you DO NOT TOUCH IT until it is dry.
When the decals are thoroughly dry, in a day or so, gently wash the model to remove any "bath-tub ring" of decal solvent around the decals. Then Dull-coat the model, weather, etc.
The quality and variety of cast metal detail parts available today is nothing short of astounding. The intricacy of detail present in many of the better quality efforts leaves little to be desired, However as good as these products are, proper preparation is essential to get the best possible result. Below is the best general preparation process:
For the primer I have r\frequently used plain automotive primer in the spray can.
It generally works ok, but is hard to apply in the very thin coats necessary for
highly detailed parts. In this situation airbrushed primer should be used.
This is a good all-purpose technique for anything that needs to appear built into the ground, but which you still want to be removable. I use it on most of my structures, and works very well for bridge abutments and the like. It allows you to push the object into the soft plaster, let the plaster dry, and then remove the object leaving a close fitting impression. This works best if the object to be planted needs to mate a very irregular surface into the terrain, or if it needs to be partially buried. A good example of this is a loading dock I did a while ago that had wooden supports holding up the dock. By using this technique the supports could be deeply buried in the terrain, and yet still be easily removable.
If you are planting a structure that has a generally square footprint, another alternative is to use the plastic wrap (Saran Wrap) method. Just cover the item with the wrap as close as possible, and plant it in the wet plaster. Once the plaster is fairly well set, you can remove the structure and wrap and allow it to dry completely. Once done, you have a fairly tight fitting impression. This doesn't work as well for complex structure/terrain matings, but has the virtue of being a whole lot easier.
I have not tried this yet, but it looks really promising. A friend of mine says that a great way to make your own lumber is to use hardwood veneer strips. These are sold in hardware stores, and are normally used to cover or trim furniture and cabinet work. They are usually sold in rolls of one or two inches wide, and about eight feet long. The rolls are very thin, being about 1" thick in HO scale. This might be a little thick for some uses in N scale, but a little sandpaper should cure that. They are usually available in all the major quality woods, such as walnut, oak, birch, and fir. Make sure you get the type that is made to be glued, not the ones with heat sensitive adhesive on the back.
With the use of a knife a strip can be cut into any size boards, and are said to make pretty good looking plywood sheets. They have a real wood grain, that you just can't simulate anyway else. My friend is using veneer boards for a structure he is scratchbuilding. Once he glues the boards onto the frame of his structure he uses a pencil to mark in nail holes. This looks specially promising for making scale plywood. I think I might cut some plywood sheets, and use them to cover up the windows on one of my "abandoned" structures.
One small problem is that since they are sold in tightly wrapped rolls, they are very curly at first. However some time spend under a book or other flat weight can cure this.