You’ll need some color film (the results depend on the film so make some research), empty film canisters, tape, scissors and a marker. It’ll be easier to get the film from one canister to another if the empty film canister still has the tip of the old film visible and available. You need also an access to a darkroom or a changing bag.
On the left is the side which is normally exposed first. On the right is the “wrong side” of the film which is exposed first when you take redscale photos.
First step: cut of the tip of the film.
Second step: align the film you are redscaling (upside down) with the tip of the old film in the empty canister. The empty canister is the upper canister in the photo below.
Third step: tape the film ends together. Make sure the tape is attached properly and it does not make the join too thick.
Fourth step: roll the film into the empty canister. This must be done in a room that is completely dark. Any light will ruin the film. You can use for example scissors or another tool to turn the center of the canister (be careful not to damage the film!) or you can turn it by hand.
Fifth step: once the film is inside the new (old canister), cut the film from the canister. Leave a short tip available so you can use the (now) empty canister to make another redscale film.
Sixth step: cut the end (lead) of the redscale film to make it easier to load the film into the camera. PS: You may want to cut it the other way around. I was blonde enough to cut the lead incorrectly and had to recut it when loading into a camera.
Note that if you don’t have an empty film canister to use, you can pull the entire film out of the canister, cut it, turn it around and tape it, and then roll back in. Do this in a darkroom.