But if I tie it first I have too much rope when I go to tie the anchor knot.
Too much rope isn't a problem. If it's a long rope, and you don't want to cut it, then it has to go somewhere, so you'll have a long rope either at the noose's side, or the anchor's side.
Here is what you could do:
- Tie the rope to your anchor point, and leave enough rope on the weight-bearing end. It should be long enough, so that it comes down to the desired height and there's still enough rope to tie the noose. If it's too long and there's excess rope, that's not a problem.
- Pull on the rope, and test whether the anchor point is secure.
- Tie a Noose Knot in the rope. Once that's complete, tie an Overhand Stopper on the tail end (or loose end) of the Noose Knot, as close to the Noose Knot as possible. To be clear, this will be the end that is dangling, not the one leading to the anchor point.
- If there's excess rope on the tail end, that's not a problem, it can be of any length, it doesn't matter. It can make tying the knots a bit uncomfortable, but it won't affect the Noose Knot functionally.
- Test the noose: put your wrist in it and pull it downwards. If the noose tightens around your wrist as you pull it down, that means you've tied it correctly. If it doesn't, then you've tied it incorrectly.
The video Forveleth linked above shows two ways to tie the Noose Knot. If the rope is relatively short, both methods work. If the rope is very long, the first one is more convenient. Note that the video does not include any stopper knots, and you need to add that as well – it's really simple. Tie it in the tail end of the Noose Knot, and tie it as close to the Noose Knot as possible. Here are some instructions:
Overhand Knot