In my case I own a set of Cokin "P" series of B&W filters, and a P series ring to fit 58mm threaded lenses. My other film camera is a Mamiya 645 medium format, and while it shoots great pictures it doesn't do wide format landscape. My family purchased the new Holga 120Pan, which shoots 6x12. I was so excited to get it for Fathers Day this year! They ordered it from Freestyle Photo, which is a great resource for analog photography.
When I received my new Holga, Freestyle was kind enough to include a book with the history of the Holga. There was also a lot of info on accessories, where it stated you can "Firmly twist the step-up ring into the pliable plastic lens of the Holga." (quoted from page 33). I prefer not to do this because it is far too easy to get the ring on crooked, or not on firmly enough and have your expensive filters fall to the ground. My process involves removing plastic from the inside the front of the lens (about 1/2 a millimeter) in order to press fit the step-up ring into it.
The steps below will take you through gathering your tools and supplies, disassembly of your Holga (the steps will also work for a standard Holga), fitting the step-up ring, and re-assembly. I highly recommend removing the lens to avoid breakage or scratching.
P.S. Hooray for my first instructable! I hope you enjoy it!
Added June 27th, 2012:
IMPORTANT! : I just received my Holga Shutter Release for long exposures and found out that it won't fit over my step-up ring... OOPS! Not a real big issue, though. If you go to put yours on and it won't fit, just take the lens off and put the Shutter Release on, then re-install the lens.
P.S. I just shot part of my first roll of film with the camera and used my polarizing filter. I'll post a few scans after I finish the roll, which won't be until Friday the 29th.
Step 1: Gather your tools and supplies
1. A small screw driver
2. Thick tape (I used Gorilla Tape - available at Lowes or Home Depot)
3. An RX bottle (mine was a T-20 size - located on the bottom of the bottle), or a similarly sized strait sided cylinder. It must fit close inside the Holga's lens
4. Sandpaper - 220 grit will work the best
5. A hobby knife
6. A flat surface
7. A clean towel - not shown
8. A can of compressed air
9. A step-up ring to fit your filters.
10. Epoxy of your choice (if you intend to attach the step-up ring permanently) - not shown - 5 to 20 minute cure times recommended
11. Mineral Spirits (if you will be using epoxy)
12. Paper towels (again, if you will be using epoxy)