If you are referring to ready-made curtains, hopefully you have allowed extra width for gathering. I usually buy twice the window width, or more, any less will result in a poor gathering 'sticky-out' curtain.
Starting at one end of the curtain tie a strong knot between the strings (could be two or three of these). Then attach to a door knob and pull the curtain, gathering as you go, intil the curtain is half the width of the window you are covering.
Repeat with the other curtain, but knotting at the opposite end. Tie off with knots that you can undo, this is for laundering the curtains as they will need to be flat. I believe you can buy a plastic thingy to tie-off and curl the surplus string into, if not dont worry, use the excess and coil after knotting and poke it into the header tape (this will be at the wall edge of each curtain, not the middle where they meet.
Space your curtain hooks evenly, remembering to hook the wall ends onto the permanent eye, so the curtains dont meet at the middle and leave the sides bare.
A useful tip, for graceful curtains, is to draw the curtains open manually fold into pleats that you would like the curtains to fall into when open, then tie scraps of fabric at intervals between the top and bottom to 'train' them. Leave the scraps in place for a day or two before removing.
You should now have gathered drawn curtains at night, and well trained pleats of curtains during the day.
Good luck.