1) buy one with a good brush (fine, with straight bristles and a good natural point)
2) arrange dressing table/mirror/se
3) tilt head back so that you are looking down your nose straight into the mirror with your eyes half closed
4) hold eyeliner brush like a pen
5) steady head with relation to face - depending on hand size/finger length/brush type you can either rest your chin on your hand or rest the tip of your little finger on your cheekbone - you'll need to practice a bit to find a comfortable position that leaves the brush pointing down at a slight angle so it clears your eyelashes and so you can still see the end of the brush clearly in the mirror. Keep both eyes open so you can judge distance properly
6) with the brush pointing slightly down and the tip right up against your top lashes, about 1/4 of the way along your lid (1/4 of the way out from your nose) sweep to far end of lid, staying close to lashes. Push a little harder over the middle half of the sweep, then gradually take the brush away so it is barely touching the skin by the time you reach the end of the line (tilt the line up a bit at the end if you are planning to go a little beyond the lash line )
And there you go! Piece of cake. Go away, try it. You'll look marvellous. Sorry, marvellous-er.