I am assuming the sticker has some type of adhesive back appropriate to
decal type stickers.

My son has run a highly successful  automotive window tinting and
commercial window tinting operation for 17 years. He uses Johnsons Baby
Shampoo exclusively. Buys it by the case. The shampoo, when mixed with
water , gives a surface for the tint to be floated on, then the tint can
be moved for positioning. It softens the glue on the back of the tint,
and then the front surface of the tint is sprayed and this allows the
tint to be squeegeed into place without the squeegee tool damaging the
surface of the tint. The pressure of the squegee forces the excess
shampoo mix and glue out from under the tint. The surface is then
cleaned with more shampoo and squeegeed again to clean and its done. Dry
the surface with a T-shirt or other soft cloth. No heavy rubbing ---it
will disturb the tint/decal. Just blot to dry. The glue/shampoo mix
under the tint film will dry very slowly and will dry very clear. No
bubbles.

Same principles apply to many decals, although there are some out there
that have a stickyback adhesive. These would not be appropriate for the
shampoo treatment.

Two tablespoons  in a quart of water is plenty for a small job.

John Rodgers
"88GL Driver wishing he had ordered the stickers.