Littlefun is correct, but a little more explanation.
Use an empty household trigger spray bottle (clean it out well first though). Fill with water and 3/4 drops of washing up liquid. Mark out area with masking tape with pen marks for centres etc so that you know where your decals will be going (Essential step). Degrease surface area for decal, with 'Panelwipe' or IPA alcohol. Trigger spray a mist of the water/soap solution. This is to wet the surface and break down surface tension. Place the decal in position and slide about to position. It will initially feel that the liquid is preventing the decal from sticking, but smooth out from the centre with the back of the hand to squeegee out water/soap mix. I don't use a credit card or similar as I found that the edge can easily scratch the vinyl. (HDIK?). Leave for 24 hours min if you intend to clearcoat over the decal.