Buy FABRIC paint and it won't be an issue.
Don't rely on an applicator tip to 'draw' if you buy a brand that comes that way. Air pockets inside the tube create messy splashes when they happen.
Painters tape can be used on fabric. It's a great tool, use it.
Paint along the tape and from tape toward your area, but never area toward the tape. On fabric tape doesn't seal as well, thus brushing paint towards it can give you a less crisp edge.
Yes practicing on not-your-project is ALWAYS a good plan. In this case an old tee shirt will suffice.
These are things we can't really tell you that you will learn from practice:
- how much or little the paint soaks in or sits on top of the fabric.
- how steady your hand is.
Is your fabric/ garment stretched enough or is the fabric shifting around under your brush?
- is the place you are working sufficient for this project?
- are there human or environmental factors that will come into play?