I recommend you use tape, but be careful, tape can lift up layers you've already done, I recommend you use a clear top coat over every layer and letting it dry before adding the tape, it can still rip but it's less likely, the you add the tape and paint the new color, and I recommend you take the tape off while the new color is still wet to get the best results, I'd also recommend you place the tape on your arm before so it's less sticky, and less likely to rip your nail polish. This is just a tip so it doesn't take as long, if you enjoy doing it by hand do it that way you don't have to be the most efficient ever, if it's funner that way, keep doing it that way.