Find a fine-tipped permanent marker (or my favorite for that extra feminine touch for a sissy: a lipstain) and pick out a design you like. For sissies with limited artistic experience, a simple shape will work best (the shoulder star like Bianca here, a Venus symbol, a bow). In a pinch, a short word also works well. Capital letters are easier to write neatly - even if you've been practicing your girly handwriting, writing on yourself may be difficult. If you can't think of anything else, labelling yourself SISSY would be just perfect!
But you don't want to hide your art away!
Once you're done, you'll have a neat little girly reminder that's just a wrinkle or ruffle of clothing away. A raised sleeve, a lowered sock and you'll be able to show off your girly art!
Most permanent ink will wash off within a few days (depending on your marker, it may become much less visible after just one wash - feel free to touch it up as you see fit). If you have sensitive skin, test out a marker first to make sure you won't have any unfortunate rashes or uncomfortable events to ruin your artistic experience.
Most importantly - have fun and express your inner sissy!