The method described by Microscale, is to use 'Sol' on the decal before application, leaving it a minute or so to act, then apply to the model, adding 'Set'.
The decal is supposed to wrinkle, as this is part of the softening and setting process, and should not be touched until it has set.
However! Many years ago, I came to the conclusion that using both, apart from (seemingly) having little benefit, took time and could get complicated. ever since, I've only ever used 'Sol', for both applications, as this is the stronger acting agent, with 'Set' being designed more to pull down and 'stick' the decal.
A lot depends on the decal itself, with the ink, carrier film and backing paper all playing a part in the 'performance' of the decal. Some thinner decals may only need brief soaking, and just a little 'Sol', others might not require 'Sol' at all, whilst thicker backing paper, and 'heavier' decals, nay require longer soaking, and more 'Sol' being left on the decal before application, and perhaps more to aid positioning and 'draw down'.
All of this, of course, assumes that the surface to be decalled is clean, and has a good, smooth, gloss finish. Greasy finger marks, not always immediately evident, a semi-matt or poorly painted finish, will all affect the performance of the decal, no matter what decal solutions are used.
I've found that the best results are on a surface with a gloss paint finish, as opposed to a gloss clear coat, and this is certainly the case with Xtracolour enamels, gloss paints in the correct camouflage shades, made especially to make decal application easier and more positive.