Hi there. Okay - I'll try to give a step-by-step guide and assume you haven't done this before or seen it done.
First iron the curtains and leave them to air. If they have only a few creases don't scrimp on this step - they'll just irritate you when the creases don't drop out!
Use a tape measure - dress making one is easiest because it's totally flexible - and measure the track or pole. Halve this figure and this is how wide each gathered curtain needs to be. I always then add a couple of centimetres to this figure to allow for a slight cross over in the middle so you don't have a gap when the curtains are closed. Your curtains should be at least one and a half times the total curtain track length - preferably twice the length, for the best look.
Now have a look at the curtain header tape. You'll see three horizontal cords running across. At what will be the outside edge on each curtain of a pair carefully unpick the ends of these three cords only as close to the header tape end as possible. Holding the three cords together carefully pull them gently until they are just long enough to knot together. Knot together securely and tightly.
Repeat this procedure on the inside edge of the header tape unless the cords have been extremely well securely sewn in.
Sometimes only one end of the gathering cords can be freed - this end is the outside end of your curtain for the neatest look.
If you haven't knotted the cords at both ends, when you start to gather the curtain you will pull them out. It's a right faff to fix.
Okay - with me so far? Back to the outside edge of your knotted header tape cords. Holding the knot in one hand and the header tape/top of the curtain in the other pull the curtain away from the knot. It will start to gather. Ease the gathered section along so it isn't just bunched up in one place. Once you have gathered it part of the way, measure with your tape measure and see how far you need to go. Don't worry about how even it is at this stage, you can sort this out afterwards.
When the gathered curtain is the correct width you measured earlier, you need to tie the cords together to stop them from moving. When I do this I don't pull the ends of the cord all the way through the knot, I leave a large loop as this helps tidy up the cords and makes it far easier to undo this knot when you take the curtains down for cleaning.
Even the gathering up along the width of your curtain.
Now count how many gilders are on the track and halve them. Push half to one outside end of the track and the other half to the opposite end. The halved number of gliders plus one end stop (fixed to the track at each end) gives you the number of curtain hooks you'll need.
Hold your gathered curtain up to the track and look at which of the three horizontal thick bands in the header tape should take the hooks. You want to cover the track with the top of the curtain while making sure there isn't a gap between the curtain and the window sill, for example.
Space your hooks out evenly - I put one in the outside end, one a couple of spaces from the inside end (so you can get the curtains to meet up properly in the middle) then space the rest out in between. This is quite easy as you just put them in every 3, 4 or 5 spaces for example. Quite tricky to explain, but easy when you see it. For very wide curtains it helps to put a hook in the middle point of the curtain before you space the rest out.
To put a hook in, poke the long end of the curtain hook through the horizontal band from below. Once it's through, pull it up and towards you so the short end wraps round the band. This stops the curtain hook from coming out.
Now tidy the long ends of the gathering cords away by tucking them into the gap behind the header tape at the outer edge. Do not cut them off whatever you do!
To hang the curtain start at the outside end and put the end hook through the end stop. Then work along putting the next hook in the first glider, working along until all the gliders have a hook in them.
Repeat on the other side.
Pull curtains shut. Voila!
Phew - that was loooooooong.