Hervey says:

Raymond,
I will explain what I do but I recommend you google search the topic also.
To start you need a clean and glossy surface to start with. Clean means no fingerprints or dust. Glossy means if you have not painted the subject you wish to decal with glossy paint then apply 2 coats of clear glossy varnish over the model.
Once the varnish is dry start cutting out the parts of the decal sheet you wish to put on the model. I cut out all the sections I need to do one side of the model so that once I start applying I can complete one side without pausing to cut anymore sections. When cutting out the sections cut as close as you can to the letters, lines or numbers on the exterior of the section
Prepare a small container with room temperature water and a piece of folded paper towel. Also keep additional water and an eyedropper handy.. Wet the paper towel then select 3 or 4 sections to start the process. Soak those 3 or 4 sections, one at a time, in the container by holding them under the surface with tweezers for 10 to 15 seconds. Remove the section from the water and place them on the wet paper towel. While the glue is continuing to soften now is the time to “paint” on a coat of Micro Set where you wish to install the first section. As you proceed keep the paper towel wet using the eye dropper.
Prod the decal to see if has released from backing paper. Just move it a bit to give you a piece of backing paper to lift the section to the model. Slide the decal off of the backing (I use a toothpick) onto the model and the Micro Set. Use the toothpick to align the decal. If needed add more Micro Set to keep things flowing. Work your way through all the sections using the same method.
Once all the sections are on and the Micro Set has dried gently apply Micro Sol to all sections. This softens the decal to allow it to snuggle down to the model. Once applied do not touch the decal until it has dried as the decal is very soft. If it has not conformed to the shape of the model reapply the Micro Sol.
If that is not enough try Solvaset which is Micro Sol on steroids. Once all this is dry carefully look at the decals to see if any air is trapped between the decal and the model. If there are air bubbles poke a hole in them with a sharp pin being careful not to tear the decal. Put Micro Sol on these spots and allow to snuggle down and let dry.
Once dry wipe the model with a wet facial tissue to remove any sign of the setting and dissolving solutions . Once dry make sure the model is dust an fingerprint free then apply a couple of coats of mat varnish.
You are now done decaling and can move on to weathering.
A couple of additional things. Make sure your work area is well lit and work with the model laying horizontally so that gravity doesn’t cause a decal to move out of alignment.
Good luck.

David Stokes says:

Decals or waterslide transfers are easy to use if the right technique is applied.

Firstly, leave the decal in warm water for only as long as it needs to slide the decal off the backing paper. Any longer and the decal might disintegrate, fall off in the water, fold or crease.

Secondly, prepare the site by removing all fingermarks, oil and dirt. Spray the area with gloss varnish, allow this to set and then apply one of the commercial settling agents. The gloss helps the decal to slide into place.

Using a small paint brush dampened with water to slide the decal off its backing paper onto the model. The brush or a toothpick can be used to manipulate it into position. Do not use tweezers or anything metallic as these will tear the decal.

Now wipe off excess fluids with the paintbrush or a paper tissue and apply a setting agent (Note I have said setting and settling agents – each has its use) and allow to dry.

Shoot a thin coat of gloss clear over the model to hide any decal film or “silvering.