Bracelets
The starting project for most people, and the one where measuring matters most, because two centimetres out is the difference between a bracelet and a bangle.
Measuring
Measure the wrist over the bone, snug but not tight, then add about twenty five millimetres for comfort. For a side release buckle, subtract the length of the closed buckle from the total; what remains is the length to fill with knots.
Cord length
A rough rule for a cobra: thirty centimetres of cord for every finished centimetre, per pair of strands. Cut generously. A short strand cannot be extended invisibly, and offcuts are useful for key rings.
Choosing a knot
Cobra for a first piece, fishtail for something slimmer, trilobite for width, and a sliding closure where no hardware is wanted. All four are covered in bracelet kits with cord and buckles (https://www.123paracord.co.uk/diy-kits/bracelet-sets/).
Finishing
Melt each end, press it flat against a hard surface with the back of a knife rather than a fingertip, and let it cool before handling.