Measure the base ring first and the cage second. The ring is the fit-critical part. Too loose and the whole thing slips off, too tight and it restricts circulation, and neither is salvageable by liking the look of the cage. Measure, then choose.
Go shorter on cage length than feels right. The cage should be shorter than flaccid length, not matched to it. This is the least intuitive rule in the category and the one that produces most of the returns.
A kit with several ring sizes is worth the money. Fit is genuinely hard to predict from measurements alone, and a set that includes multiple base rings lets you find it without a second purchase. It is why the resin kits remain the standard first buy.
Start in resin or silicone. Metal is the goal for a lot of people and a terrible place to begin. Light and warm first, heavy and cold later.
Build wear time in stages. An hour, then an evening, then a night. Skipping to overnight on day one is how people decide the whole category is not for them when what was actually wrong was the schedule.
Numbness, color change or pain means off immediately. Not soon, not at the end of the scene. Immediately, with no negotiation and no exceptions, and that rule belongs to the wearer regardless of who holds the key.
Have the conversation about the key. Who holds it, what the release conditions are, and what happens in an emergency or if someone is unreachable. That conversation is the actual product here. The cage is just the object it happens around.