ItsAqua easiest way is to use a string or if you have a cloth measuring tape measure around your forehead above your ears keeping the tape level with the ground have somebody else do it if you can if you do it yourself make sure you do multiple measurements to ensure that you’re getting the correct measurement if you look at like RevZilla they’ve got a great video that explains how to best do it you also want to have someone take a picture of the top of your head and that will help you to figure out the overall shape whether you are a oval intermediate oval or elongated oval and that will help also guide you to the right shell shape and prevent you from having a lot of uncomfortable moments and wasted time by having a completely incompatible shape and trying to cram your head into it which just doesn’t work very well.
I totally respect your desire to do things however you want to do them and whatever order you wish to do then, not trying to push anything on you and only trying to help so if you are interested in the helmet however let me know because I can guarantee that it will be a deal better than you will find anywhere else that’s just how I roll so I’d rather see somebody safe and happy rather than make a couple extra bucks so see what you got measurement wise if it happens to be a medium which is going to be at 57-58 cm get back at me 🤙👍