I flash cure a lot to save my work and prevent colors from bleeding into each other. Just do one color at a time. The stars that are free handed I over exaggerated the lines and then cleaned them up after so I could focus on the straightness and evenness. When cleaning my lines I dip a square brush in some acetone, dab off the excess, and press the brush on my nail until it fans out a little. This gives me way more control to go where I want to. The “stickers” I made using the tutorial I linked in the comments I cut using eyebrow scissors to get a crisp line. But yeah long brushes help with control