There are various ways and tools to measure your ring size. We use this universal band for consistency in the studio. When measuring your finger, make sure the band is able to wiggle past the widest part of your finger. Avoid measuring your finger after a salty meal or when it's hot, that's when your fingers can swell up. It's better to err on the side of snug than loose.
If you order a custom ring, I'll send you a complementary sizer.