The whole trick is teaching balance on two wheels.

The best way, IMHO, is to use a scooter, like a razor or similar. Training wheels are counterproductive because they teach how NOT to steer a bicycle; keep the bike upright, turn the bars toward the inside of the turn, just like a car or a tricycle. This is NOT what you want them to learn. All you can learn with training wheels is how to pedal and getting accustomed to the position -- maybe useful for a tyke, probably not useful to an adult.

In contrast, a scooter has the identical dynamics as a bicycle, but in a far less threatening presentation, because you can step on or off anytime, and can balance at a walking/jogging pace.

Start in a straight line, slightly downhill, like a smooth driveway or better a walkway in your local park. Consider wearing elbow and knee protection, and try to balance for 4-6 foot increments. Then work up to coasting further and adding kicking (propulsion). Then work on gliding turns and 8's. When - and not before - she is COMPLETELY comfortable on the scooter, it will be time to try a bicycle --same sequence; coasting down a driveway, then adding pedaling, then turns. I'll bet she will just start riding away.

An adult who hasn't learned to ride a bicycle may have anxiety issues regarding the bike. If so, make sure you spend lots and lots of time on the scooter - like maybe 2 or 3 weeks - to where balancing is completely second nature, because anything that is not completely unconscious and reflexive will go out the window when the anxieties show up. You'll know when she's ready because she'll be completely competent and playful on the scooter, with no signs of anxiety.

A tandem is an option, because the stoker really doesn't need to be balancing. Having hauled kids around on bike seats i can suggest it is more work than you want to deal with, to have a stoker who doesn't know how to balance. I don't think an adult will learn how to balance by riding on a tandem, but it may be a step in reducing anxiety. Then again, maybe not. I don't think I would use a tandem.

FWIW, i haven't done this with an adult, but did use a scooter to get over the "hump" with my reluctant 6 year old. After becoming competent on a scooter, riding a bike was a non-problem.

i am an adult and i've been wanting to ride a bike. Never been on a bike before. I attempted few years back and end up sending my ex to a hospital.
two years ago i started to search online for someone to teach me and came across this wonderful training - he was in Palo Alto, California back then but i know he moved to a different state. Here is what he had me to and it worked for me.

first he took off the pedals from a regular size adult bike and he taught me how to balance but walking the bike as fast as while sitting on the saddle, when keep a steady speed, get my feet off the ground and learn to balance. yes it took me quite a while to get the gesture and learned to balance. but it worked! think about it... i ride on my own speed and it's definitely fast enough to get my feet off the ground and yet in a controled way - the rider builds their confidence knowing they can easily stop any time by placing their foot on the ground. i was practicing in a school yard with a slight downward slope so the rider does not have to worry or fight a climb.

the second step he has done is put back the pedals and do the same exercise but this time learn how to step with your strong foot to on the pedal. again it is controlled - the rider goes on their own speed. in this case, the only additional step to learn is to walk the bike with your feet far away from the tires to avoid hitting the pedals. we were also practicing the actual movement of stepping up to the pedal by practicing the same movement over a side step...

slowly, he tought me to walk the bike as fast as i can, mount the pedal (with your strongest side, in my case, i thought my left foot was stronger but i realized mounting the pedal with my right foot came in a lot easier than the left foot) so explore mounting the pedal with either side until you get it. the additional step here is to not only step up with the foot, keep your balance, but also learn to put your other foot and do the push forward.

yes it took me about three weeks with a couple of hours every sunday to get there.... i started practicing on my own in a parking structure over the weekends and here i am - of course i can not claim i am a racer or anything to that level, but i slowly build some confidence to ride on the street. and YAH last week i rode for over 20 miles...

never give up folks - if i can do it in my 40ies, any one can!

cheers and good luck

I sold bikes for years and a few times each year someone would tell me they need to learn how to ride a bike.

Important: Get a helmet and gloves before. Gloves are great if you fall. I would add knee and elbow pads too.

Step 1 - This is not the only way, but it has wroked. Keep the bike simple, very sinple, if you have a cruiser start with that. REMOVE THE PEDALS and lower the seat so they can reach the ground from the seat - a super comfy seat. On a nice day , go to big, flat empty parking lot and have the person push themselves around the parking lot until they gain confidence. Next install the pedals and have them try riding.

Step 2 Same big, flat parking lot, try a more complex bike and see if they can stop and shift. Please teach them the front brake is used in tandem with the rear. Not to hard on the front.

Step 3 If they can get around that parking lot, the next step it crucial. On a nice day go to a paved flat car free bike trail and let them get going. Stay to their side - many people do not realize biking is fast and there are others on the trail. Stay on your side.

Step 4 I would repaeat step 3 many times before any big climbs or street riding.

Thanks for reading and check out my website www.gadgetbottle.com

Steve