Installing an inner tube

First, check whether the tire itself is damaged. If it has a large gash, and you can't get a replacement tire right now, you might be able to patch the inside using the same patch kit you use for tubes. If you don't have a patch kit with you, temporarily "patch" the tire. Find a piece of old tube, paper money, or candy bar wrapper. Place it inside the tire, covering the damage. Once the tube is inserted and inflated, it will hold your patching material in place. The patch will hopefully protect the tube and prevent it from bulging out of the tire.

Are you worried that your tire could go flat again from a future puncture? If so, buy a tire liner from a bike shop and insert it before installing your tube. A tire liner is a long, narrow, flat piece of flexible plastic that you put between the tire and tube. If anything pierces your tire, the liner sometimes will stop it from also piercing the inner tube.

If your replacement tube has never been used, pump a little air into it. That will make the tube unpack and assume the correct shape.

Bicycle flat tire fixing - new inner tube Bicycle flat tire fixing - presta valve
Bicycle flat tire fixing - locking pump to valve Bicycle flat tire fixing - pump tube

Over time, inner tubes sometimes become stuck to the tire's inside. This makes it harder to remove the tube when you next need to fix a flat, but that's not really a big deal, in my experience. You can prevent sticking by coating your new tube with some kind of substance that prevents bonding. Talcum powder might work, but I have not tried it.

Once, I happened to have a cut section of an old inner tube, shown below. I noticed that it had white powder inside. I assumed that the powder was intended to prevent the tube from sticking to itself when it's tightly packed. So I rubbed this powder over a tube I was installing.

Cut inner tube as source of anti-sticking powder

Once you have the tube ready, pass the valve through the corresponding hole in the rim. You will need to hold the tire aside to get access to that hole.

Bicycle tire - valve through hole in rim

Start pressing the tube into the tire. You may have to slide parts of the tube within the tire to make the tube rest evenly all around the wheel. Make sure that the tube is not twisted or kinked.

Bicycle inner tube - inserting into tire

Using your fingers (and a tire lever when you must), force the tire bead back inside the rim. When using a lever, be careful to not pinch the tube.

Put tire within rim edges using bicycle prying bar

Check that the tube is evenly seated within the tire and rim, and not pinched by the tire bead. Also make sure that the valve points to the wheel's center. Inflate the tube slightly and doublecheck that it's still seated OK.

Check for pinching around newly installed inner tube

Lay the wheel flat on the ground. Make sure the tire's position on the rim looks consistent all around. Pump the tube full of air, as close to the pressure printed on the tire as you can manage.

Reinstall the valve's nut and plastic cap, if it has those.

Screw on nut for bicycle tire valve

Mount the wheel on the bike. You're done!