Find thread that is the same color as the button. If the button is missing, search the inside of the shirt for a spare. If you don't find a spare, purchase a button as close to the original as possible.
Cut a piece of thread several inches long and insert the thread through the eye of the needle, this creates a loop. Then pull the thread though until both sides are of equal length.
Next tie a knot on the ends of the thread to prevent it from slipping through your needle when sewing.
Turn the shirt inside out. Insert the threaded needle into the inside of the shirt where the button will be placed.
Carefully guide the needle through the shirt pulling the thread all the way through until the knot at the end reaches the shirt fabric.
Pull the needle and tug the thread three or four times to make sure that the stitch will hold securely.
Next insert the needle into one of the button holes and guide the button down toward the shirt. Position the button on the shirt where it was originally located.
Insert the threaded needle into another button hole on the outside of the shirt and poke it through to the inside of the shirt, making a loop with the thread around the button. Gently pull on the needle to ensure that the thread is secure against the button.
Continue guiding the needle in and out of the button holes until the button is securely attached.
End on the inside of the shirt and tie the remaining thread into a knot. Be sure that the button is flush against the shirt.