Rubber Bands

If you are using deep frames to hold the cut out comb, #33 rubber bands work well (Thanks, Walt for that info). You can preload them on the frame by putting a few on one end. They go from the top bar to the bottom bar.
The process is pretty simple. Cut the comb to the size of your frame. Lay it in the frame, keeping the orientation the same as it was in the original hive. (Comb openings slope slightly upward) Spread the rubber bands and slide down the length of the frame, spacing them out with three or four on each frame.
The bees will cut them out when they have attached the comb to the frame. The bands do lose some of their tension over time, so they don't "pop" the bees too badly. I have often imagined the bees using them sort of like amusement rides
I still use rubber bands on some cutouts, and I like to use them in conjunction with cross wired frames. But I have made some hinged frames that allow me to lay the comb in place and close it up like a book. Do a search on here for "cut out frames" or something like that.

I like the rubber bands. I put them both horizontally and vertically. If the comb is wide I use more horizontally. If the comb is narrow I use more vertically.

Tried string and it sucks when your hands are sticky with honey from the cutout.

Tried the hinged frames, while they work and are nice, they take up a lot of room and if you plan on using them again, you will have to remove the cutout comb at some point in the future. Also there is the time factor to make them.

Thanks for the idea about preloading the rubber bands. Hadn't thought of that. That really solves a problem when your hands are sticky and the comb is warm which makes putting the rubber band underneath the comb hard.

I say that because I lay my frame down on a cutting board I always have with me. Lay the comb into the frame and cut it to fit snug. While it is still laying on the board I slide on the rubber bands across the frames. Get less stings that way, less stress on the larva and eggs and only half as many bees to deal with while do the work.