there are three types of decal/sticker commonly used in gunpla modeling:
the ever so special, foil sticker.
the most common one, comes with most gundam models. there is this type ^, and some another clear one, usually in comes in a larger set. it doesn't matter anyway, they work the same. you peel the sticker off its backing and carefully attach to it's designated area in your gundam.
next one is the etching decal, decals that are on a clear hard plastic sheet with a white backing,
very easy to use. cut out the decal you need, put the decal over the part, then etch the back of the decal with something flat like a nickel, your nails, the back of your tweezers etc. just put enough pressure to scratch off the decal without leaving pieces of the decal stuck to the backing, if there are pieces of the backing still attached to the clear sheet, put it back down and continue etching until it sticks to the piece.
very easy to use. cut out the decal you need, put the decal over the part, then etch the back of the decal with something flat like a nickel, your nails, the back of your tweezers etc. just put enough pressure to scratch off the decal without leaving pieces of the decal stuck to the backing, if there are pieces of the backing still attached to the clear sheet, put it back down and continue etching until it sticks to the piece.
the final one is the water slide decal with the dark blue backing.
easy to apply, and gives you enough time to fix it in place. so first cut it out using an art knife or some scissors. pick up the cut out decal with some tweezers and dip in water for 5-10 seconds depends on the size of the decal your using, the bigger the longer you have to submerse it in the water. after, slide it off with a cotton bud to your piece. but be careful, it could get lost or stick to other things.
and that's the last of it, thanks for reading!