Model decals

Those aren't decals. They are photoetched metal. A good way to apply them is with a drop of clear lacquer. Use a tiny brush to apply the lacquer to the back of the metal piece, then use some fine tweezers to place it on your model.

These photoetched parts are a great way to get delicate parts in a scale thickness. You may have some models with parts like this already, like seatbelt buckles or mesh grilles.

 

I saw those on e-bay too.They are supposed to be for 1:24 models but you can put them on your real car if you want.Superglue would work the best (if they don't have adhesive backing, which they look like they have)but with superglue is risky.If you apply extra it would show and there is pretty much no turning back.A lot of people use white glue since it is forgiving,dries slow and clear and believe it or not-holds pretty good.Something I like to use is lacquer clear coat.When you apply it as glue it dries invisible and hard as nails.If you really want the best then buy a model car magazine and I am sure someone in there(you will have to go through all the ads) would sell the crystal-clear super-strong adhesive that watch makers use.Most people who carry it would have it with a needle applicator which I would recommend using.If you have any other questions PM me.

 

The auction actually says metal transfer, they might have adhesive on the back already if not, try one of the suggestions listed here.