Before doing anything, make sure that you seal the model with your preferred method of sealer. I use Testors Dullcote, because that is the finish I like, but whatever you want, is fine. If you skip this step, the solvent can melt the decal into the model and bleed colors into the decal. So make sure it is nice and sealed. Next, apply a layer of the Decal Set to the area you intend to place the decal on. This will prepare the surface for the decal. I usually do this while warming up my water. You can then cut out your decals, and fill up the cup. If the decal is going to be on a rounded surface, I will generally slit them so that they can apply more easily.
Once your decal is ready, soak it in the warm water. Cold is okay, but warm water will accelerate the time needed until you can remove the decal from the carrier film. Some carrier film takes longer for the decal to come loose than others (like those from Forge World), and if you want to protect yourself from fishing in your water, you can place the tweezers so that they cover a small part of the decal as well. Then, using one brush and the tweezers, align your sheet with the area on the model, and slide the decal onto it. Be careful here, as the transfer can fold onto itself, or become misaligned. If that happens, just make sure your brush is wet and play with it until you get it where you want. Smooth out any bubbles or wrinkles as well.
Finally, apply another cut of the Decal Set on top. This will help the decal melt onto the model. Once dry, seal the model again and your good to go. For those wishing a more uniform look, now is the time when you can paint over the transfer and get it looking nice and neat.