If you buy suede boots you should have no problem. With leather or synthetic, the paint tends to scuff off pretty easily. If you just walk carefully, this is usually ok too.
To get clean straight lines, stuff the boots with balled up newspaper, then either watch some youtube videos about automotive pinstriping and use those techniques, or lay down masking tape on either side of the line before painting. Peel the tape up when the paint is slightly but not completely dry.