I've always found using tape to make clean lines when doing nails to be really, really difficult. Usually because my tape was too sticky or something like that! But I just came across an awesome tip!
Use those reinforcement stickers that are meant for paper with holes to make the curve!!!! So clever, right?!
Here's the full video where I found the tip, but there's not much more to it!
Just make sure you let the first color dry completely before putting the stickers on, pull them off SLOWLY, and finish with a clear coat!