There are many, many different methods. Some involve expensive and complicated stretching apparatus, others just some surgical tape and a lot of patience.
The easiest, cheapest way is also the least effective and could take many years even if you do it 24 hours a day every day. It’s called cross-taping and works like this: Roll your remaining foreskin up as far as it can go and tape it in place with two strips of surgical tape (or band-aids, but those are much more expensive than buying just the tape), one strip across the other, over your glans. Put a small piece sticky-side out in the middle so the tape doesn’t stick to your glans and will be easy to remove. You’ll have to at least partially remove it every time you pee.
The idea is to apply constant gentle tension on the foreskin, stretching it and encouraging it to grow to compensate. Cross-taping, while it does apply some tension, doesn’t do a very good job of it. Thus, it will take a very, very long time if it’s the only method you use. It’s also hard to apply equal pressure to the inner and outer foreskin when cross-taping. You want both sides to grow at the same rate and cross-taping applies almost all of its force to just the outer, where it’s stuck.
The most popular and also the most effective taping method is called T-taping. It will still take years, but much less than cross-taping. It first involves a bit of setup: get an elastic band, the kind used in clothing like underwear. The wider the strip, the more comfortable it will be to wear. To one end, either tie a loop of fabric big enough to fit around one of your legs, or an alligator clip to fasten it to your pants.
Now, take two strips of surgical tape long enough to wrap around your penis, but not quite the whole way–you have to leave room for your frenulum*. Fold them in the middle and tape the two pieces together, forming a T-shaped strip with the sticky side on the top of the T.
Once you’ve got your T-strips and your elastic, get an erection and wrap the T-strip around your penis, with the vertical part of the T over your scar line. Wait for the erection to subside, then grab the flange that’s created and pull it up as far as it can go. Stretch the elastic strap and tie or clip it to the flange. It should be tight, but not so tight as to be painful.
T-strips apply great tension and, after a couple days, you get used to the strap and forget you’re even wearing it. This method does have its draw-backs: it takes longer to apply and greatly inhibits movement. If all you do is walk, climb stairs, or sit at desks, you’re fine, but anything requiring more leg flexibility than that is right-out.
Once you’ve grown out your foreskin enough that it can stretch completely over your glans, you can ring-tape. This is about as effective as cross-taping, but is by far the quickest and least cumbersome taping method: roll your foreskin up and tape around the top in a circle with a small strip of tape.
I’ve been doing a combination of T- and cross-taping** for the last five months, three weeks, and five days. My foreskin is greatly stretched, but there’s only been around 4mm of new growth. I figure it will take between two and three years over all. I try to do it at least 23 hours a day with no more than one day a month off. It doesn’t take much incentive not to cheat, though. After a month or so of your glans being covered, the skin thins and nerve endings move nearer to the surface. It’s most uncomfortable having it rub unprotected against fabric.
I’m assuming you’re through puberty. I’m not exactly sure why, but everything I’ve ever read about foreskin restoration stresses not to start until you’re completely through puberty, which is why I waited until I was 22 and figured I was good and done. You’ll need dedication and have to truly want it. It’s going to be many years of non-stop work.
*If your frenulum is intact, your golden. If it’s been severed, there is no non-surgical way to restore it. I have mine, but I know others have restored their foreskins through taping then afterward had a new frenulum surgically created.
**My way of cross-taping is different from the standard method. Instead of two strips of tape, I start with six strips and two small pieces. Fold four half-way over on themselves and make tiny Ts. Stretch your penis out or get an erection, whichever is easier at the time, and stick the four Ts on at the scar line, just like in T-taping. Put a small piece of tape on the top of two of the flanges with half sticking off, the sticky facing towards your body. Pull up the flanges and stick the flange on the opposite site to the other half of the tiny piece, then tape across the top. Do the same for the other pair of flanges. This corrects for the unevenness of cross-taping by applying tension to both sides at once.