What is the best way to attach the leather strap?

The easiest way is to attach the D-STRAP to the bike while it is still on the ground. So the ground prevents the front wheel and handlebars from turning.

Can I just order size L to be on the safe side?

This is not recommended as there may not be enough tension if the leather strap is significantly too long. In any case, you should read and follow our Fitting Guide before placing your order.