Be sure to see how the thread is passed through the holes on the garment's other, existing buttons so that it does not look different on the button you sew yourself. See if the thread is passed diagonally between the holes as in an "X" on the front of the button, or if the button is sewn in parallel through the whole pair.

Start by cutting up a piece of sewing thread that is not too short, so that you have enough thread to work with. About a meter of sewing thread is usually okay.

Attach the thread to the button by threading the thread through two of the button holes, so that two equally long thread ends come out on the back of the button. Now bring these together and thread both thread ends through the eye of the needle. Sewing with double thread will save you a lot of time and give you a firmly sewn button.

An important detail when sewing a button is how to keep the distance between the button and the fabric. This becomes more important the coarser the fabric. If you just sew the button directly to the fabric without creating a few millimetres of distance between the button and the fabric, it will be difficult to button-up the garment. You solve this easily with the help of two taped toothpicks or matches.

Attach the button to the point where the button was previously sewn. Place the two taped toothpicks between the button and the fabric, so that the toothpicks end up between the two holes you thread through.

Use the existing buttons as a reference to determine how much thread you should use. When sewing through the first pair of holes, then start sewing the button through the second pair of holes.

When you have sewn the required stitches, wrap the remaining thread around the threads that run between the button and the garment, to create the small "stalk" that holds the button a few millimetres from the garment fabric. Finish the job by discreetly attaching the thread to the back of the garment.