Just make sure you open the package and let them sit for a few minutes and that activates them before you put them on.
I used to put them on the bottom of my sock; now I put them on the top of the sock. It doesn't interfere with the fit of your shoe, no matter what kind of shoe it is. I do the same when I x country ski, too, although it's usually below 20 before I use them for skiing.
I wish the chemical hand warmers were flat like the toe ones. It would make wearing them for riding so much easier.