Step 1 : Measure the part of your wrist in (centimeter) where you will be wearing the bracelet

Step 2 : Do not loosen/ tighten the tape as once it string up, it will be just nice

Step 3 : Fit measurements are very subjective, hence here's a range where you can refer :

Fitting Closely : Without any Gap

Comfort : Extra 1 bead = approx 0.8mm-1cm

Here's a demo video of better visualisation