Avoid the mail-order kits at all costs because you need a proper doctor checking your roots. I did my treatment through a dental orthodontic clinic
https://md.dental/orthodontics/ and the experience was flawless. They take digital 3D scans of your mouth right there in the office, so you don't have to deal with those awful messy molds. The staff monitors your progress every few weeks to make sure your bite is moving safely. My crowding is completely gone now, and my teeth feel perfectly aligned.