On the assumption that these are traditional transfer decals, you need to cut them out (fairly precisely, but not necessarily to within a tenth of a millimeter), after which you soak them in water for a short time, until the actual decal comes away from the backing paper.
You then transfer the still-wet decal to the appropriate place and, while it is still wet, carefully adjusts it to sit just so.
The plastic kit crowd also likes to use decal fluids such as Micro-Sol and Micro-Set (**LINK**) to further improve the process; I have tried these myself for the few decals I've done in recent years, and would be inclined to agree that they are rather useful.
There are any number of tutorials, both for transfer decals in general, and for decal fluids in particular on YouTube, so some time spent there might be a worthwhile investment.
Mattias