They are projection proofs. You can do 35mm, 9 frames at a time using a 4x5 enlarger. A glass negative carrier is needed. Align the 9 frames in the carrier, insert it in the enlarger and project them onto your paper of whatever size you want. Look good and works well. Very easy to do.
If you want a full roll of 35mm film done at one time you will need an 8x10 to 10x10 enlarger to do so.
Other than aligning the negatives in the glass carrier it is no different than making a normal enlargement from a negative. Use a glass carrier to make sure the negatives are flat. If not the curvature of some will make it impossible to get a sharp image of the negative.