You will need :

A soft tape measure, the kind you find in a sewing kit!

Pen and paper.

Using a soft tape, measure the widest part of the nail (sidewall to sidewall), making sure that the tape follows the curvature of the nail.

Record the size in 'mm' and repeat on the rest of your nails.

A sizing kit is the only way to guarantee a perfect nail fit, however this method is used by many with accuracy.