you could also practice on cardboard, the shiney kind like shoebox type is similar to your finished wood
did you use oil or water based poly?
you have to use oil paint if your used oil poly and could use acrilic art paint for water poly
put a few coats more poly after your decal/stencil to protect and make it blend in better, just have to apply to surface where you applyed stencil
henrys advice about stenciling is going to look more better than any decal or sticker. there many books on crafts and faux painting in library that will have a chapter on stenciling