Sorry about the blurry pictures, I did this at night.
P.S. this is my first instructable, so bear with me.
P.P.S. This modification was based on the one available at: http://shop.lomography.com/holga-backup/35.html
Step 1: What you'll need
A Holga camera
Long, thin rubber bands (2x)
Cardboard (or any kind of padding material)
Step 2: Fix the Reel
Wrap the rubber bands around each edge of the take-up spool so that the hole is roughly the width of 35mm film. You can adjust the size by fitting the exposed end of your film into the hole.
Now place your spool in your camera. Use cardboard (or padding) to get it to fit in snugly.
Step 3: Insert Film Here
Push some through the spool's hole and twist the knob a few times to get it to take.
You'll need a lot of cardboard (or padding) to get the film canister to stay where it's supposed to. Put a bunch above and below it to get it to stay in place.
Step 4: Blackout!
Wait... what's this? Your viewing window for medium-format film is going to expose your 35mm film!
Fix this problem with a little electrical tape. Cover up the window and any other visible cracks that might let too much light in (except the shutter, we'll need that later).
Step 5: Take Pictures
NOTE: Since there's no way of knowing what frame you're on, use the "34 click rule." This means that, after every picture, turn the winding knob and count 34 clicks. This gives you enough room on the film so that your pictures don't overlap.
ANOTHER NOTE: 35mm film is considerably shorter (as in height) than medium format, meaning that your pictures will be smaller than the viewing window. Solve this problem by setting up your shots as if the top 20% and bottom 20% will be cut out of the frame (because they will).
Step 6: Unloading!
This is kind of tricky the first time: Let the film fall as you hold the canister in your hands. Find the little round knob at the top of the canister and turn it so that it retracts the film back inside. Reel it all the way in.
If you plan on developing this yourself, the have at it. Otherwise, take your film to a photo place and get your pictures printed. It's best to do it yourself so you can make a contact sheet and stuff, because most non-pro labs will just print your pics automatically. Because we're using 34 clicks and not standard automatic reeling things, your pictures may be cropped or lost completely. At least make sure you get negatives.