Find a relatively flat, non-rubberized surface, as the adhesive will not stick well to anything rubberized. While keeping the ModMic attached to the Base Clasp and without removing the adhesive backing, place the base clasp on the intended spot and ensure that the microphone is going to be pointed towards your mouth. Rotate the base clasp if needed. Wipe the surface clean with the included alcohol pad, remove the backing from the base clasp, and hold firmly for 30 seconds. For best results, let it cure for 1 hour before attaching your ModMic.
For difficult or mesh surfaces, you can try warming the adhesive with a hair dryer or similar low heat source. Be sure not to burn the adhesive, yourself, or your headphones.
For the visual learners, this video is what you’re looking for.
If the 3M adhesive has worn out, washing the 3M surface with soapy water (however strange that seems) and heating the sticky surface can renew some of the original stickiness.
If you've already tried the soap and water and heat solutions and the pads still won't stick, you can replace the 3M adhesive on the Base Clasp.
The existing gray foam on the Base Clasp comes off in one piece (with a bit of force from your fingernail; no tools necessary), and leaves little residue. Any extra can be cleaned off easily.
In rare cases, you might need a stronger adhesive.
The above links are for Amazon US; availability in your region will vary.
(Again, the existing gray foam on the Base Clasp can be peeled off with your fingernail. No tools necessary.)
Keep in mind these are MUCH stronger than the stock adhesive we use. Since you won't be using adhesive that's included with the ModMic, you're using this adhesive at your own risk.