Easy to do, and gives amazing isolation.

But...

On my Sennheiser CX300ii, the result was strong but stodgy bass. I like it strong, but the sparkle was gone. May be to some people's taste - YMMV.

Also the standard paper hole punch I used resulted in the plugs slipping off when I pulled the phones out, leaving me fiddling for a bit to get the plugs out. Be careful you get a really snug fit around the posts.

Freezing my earplugs didn't make any difference to the cutting - most likely a different material. I have Hearos Extreme Protection.

Anyway, well worth a try!