You will need:
Your washed and ironed finished piece of cross stitch; heavy rigid card which won't bend too much when you work with it (preferably preservation quality mount board to keep your work in good condition long term), a pen or pencil, ruler or tape measure, a set square, a craft knife, lots of pins, a strong thread and a needle with a big enough eye to take your strong thread.
Step 2: With a tape measure, work out how big you want to cut your card. Ideally you need to measure your card so that it is a little bigger than your finished design, but it will need to be a couple of inches either side smaller than your piece of fabric. You will need to have a decent amount of fabric at the back of your design for the stretching - if you have too little you might risk ripping and fraying the edges.
Step 3: With the set square to make sure that you get straight right angles at the corners, measure your card and cut it with the craft knife.
Step 4: Here come the pictures to try to show you what I do next. I centre the material over the piece of card and start to hold it in place by sticking pins through the material into the edge of the card. I tend to use the holes in the aida as a guide to keeping it straight. You need to do this for two opposite edges of the card and material. I normally start with the longest edge first.
Step 6: Starting from each end, you need to pull the threads tightish - not so tight that you are tearing the edges of the material - and again meet in the middle. Tie a knot in the middle to join the two ends of threads fairly tight.
The other side should now look like this. If you realise that it isn't quite centred, at this stage it should still possible to wriggle the material a bit to centre it a bit better.
Step 8: As with the longest edges, you take a long length of thread and sew across the card to draw the two opposite edges of material together. As before work towards the middle with one length of thread, and do the same with another length of thread to meet in the middle. Tighten up the thread and tie the two ends of thread together in the middle.
The back should look like this.
Step 9: Oversew the corners to fix the material in place in a neat fold. You might also want to iron this afterwards to flatten the corner down a bit.
The finished front should look like this.
I have heard of others who bypass the whole threading thing by just fixing the material in place with high quality tape. I've never done this, as I am always wary that the tape might lose its adhesion or discolour the material (although I suppose in practice, if it does, it would be behind the frame and not visible).
And that's how I do it - although it is quite hard to clearly describe, so apologies if it doesn't make sense. Hopefully the pictures will help to make up for any deficiencies in my written description...