I consider these things to be important:

Work carefully at all times

Cut the pieces very carefully so they are exactly right

Hand sewing goes from right to left if you are right handed.

Use one thickness of thread for most hand sewing. Buttons use two thicknesses of thread.

Do not wet the end of the thread, when threading the needle, because it will make it harder if the thread swells up.

Knot the long end of the thread.

Sew evenly. Following the weave of the fabric itself, make stitches of the same size, in a very straight line. If a straight line is hard for you, you can mark a line with tailor’s chalk.

Smaller stitches hold together better than larger stitches.

At the end of the line of stitching, knot the thread.