Remove your wheel. If your tire is not already flat, deflate it. If you have presta valves (skinny valves) you will need to unscrew the tiny nut at the end of the valve and push it down.

Try pulling the tire off the rim by using your hands; however, if this does not work you can use a tire lever. Sometimes you will need to use two tire levers. After you've insterted the tire levers, slide the lever around the rim to remove the bead of the tire from the rim.

After you've removed your tire check the tire for the cause of the flat. You might see the thorn or nail on the outside of the tire; however, sometimes the thorn will have pushed through the tire and you will need to rub your fingers on the inside of the tire. Also, check the rim for protruding spokes and make sure the rim tape sitting properly on the rim.

Inflate the tube with enough air to give it shape and place the tube inside the tire. Place the tire on the rim by inserting the valve first on the rim.

Push the tube up inside the tire all the way around. Now push one side of the tire onto the rim with your thumbs or fingers. Now starting near the valve, place the other side of the tire inside the rim. Use your hands if you can. If you need to, you can use a tire lever (be careful you don't pinch your tube when you're pushing the tire onto the rim.

Check the valve and make sure that it is coming up straight out of the rim. Adjust if needed. Now work your hands around the tire and make sure the tube isn't protruding under the tire. Roll the wheel back and forth, this will set the bead of the tire.

Check the sidewalls of your tire and inflate the tube to the recommended PSI.

Now get on your bikes and ride!!!