I recently found my old pair of Shure e2c in-ear monitors minus the earbud tips. I prefer the memory foam tips that seal out all outside noise. The foam tips made by Shure can be uncomfortable -the foam feels like sandpaper in my ears- and they are expensive! $15-$20 for 5 pair. I have thought for a while about making my own out of ear plugs used in my workplace for ear protection. These earbud tips are practically free since they are made from ear plugs found in virtually any high-noise area. The best part is that when they get old and dirty, just toss 'em out and make yourself some new ones!
Metal-tipped retractable pen
Foam ear plugs
Step 1: Cut the ear plugs
Cutting the ear plugs is pretty straightforward. Just make sure your utility knife has a fresh blade in it. You want to cut the plug a couple millimeters longer than the earphone shaft that will receive the earbud. That extra overhang won't interfere with the path of the sound, but it will help the earbud stay in your ear more securely. The ear plug foam is pretty squishy, so you will have to cut them with a sawing motion back and forth, slowly pressing down until you have cut all the way through.