I do these outside on relatively flat rides (preferrably un-congested with pedestrian or other traffic, too). I've only recently started doing them, on advice from a book I'm reading (Arnie Baker's "ACE" - Altitude Climbing Endurance). Once or twice a week (OK, I'm no Regimen Nazi) I do a few reps, 2-3 minutes per leg. Just put the unused leg on the chainstay out of the way; it's a little unnerving at first but you'll get over it quickly. Shift into a fairly high gear, something where I can get about 50-60 rpm. Focus on smoothness, trying to get every scrap of energy from the stroke going into moving you forward, not flailing around.
When I first started doing these I found that one leg had really been compensating for weakness in the other as my weak leg tired out much faster than the strong one. That's still happening but getting better, slowly.