slap onto
To apply something to oneself or something else very hastily or haphazardly. A noun or pronoun can be used between "slap" and "onto." Don't just slap makeup onto your face like that—you need to learn how to put it on properly. I can't understand why this artist is so revered. It looks like he just slapped paint onto the canvas and left it at that.
slap something onto someone or something
and slap something onto place something onto someone or something by slapping. Tim slapped a sign onto Gary that said "kick me." Tim came up to Gary's back and slapped on a sign.