How do you wire 4 6 volt batteries to make 12 volts?

Assuming the batteries are identical: Mark batteries A, B, C, D

Group the batteries A and B, C and D.

Join A+ to B-, Join C+ to D-. This puts the batteries into a series connection. You now have (2) 24 volt batteries.

To place the batteries in parallel (ie: use all 4 batteries):

Join A- to C-, Join B+ to D+ and use the common terminals A-C- as the combined negative terminal, Use the B+D+ pair as the combined positive terminal.

A combination like this is known as a series parallel connection.

join the positive and negative