This is a relatively simple topic. You need to know your seed in the tableau and from that figure out who you will fence.
You get to the second day in three possible ways:
If you have a bye, it's easy to figure out your seed on the second day. It's your seed going into the tournament, possibly swapped with the next higher or lower seeded fencer (the one below you if your seed is odd, the one above you if your seed is even). So 1 and 2 can be swapped, 3 and 4 can be swapped, etc. Each pair is swapped at random, to avoid repeating bouts between fencers who always have the same seed at multiple WCs. You won't know if you've been swapped until the Directoire Technique prints the second day tableau.
If you are one of the top 16 out of the pools the first day, it's easy to figure out your second day seed. Take your seed out of the pools and add 16. Voila!
If you came up through the DEs, it's harder to figure out your seed. The 32 fencers are reordered by their seed going into the DEs (no, you don't get to keep the slot you take over with an upset) and then put into the bottom half of the tableau of 64. You should wait for the DT to print out the tableau and find yourself there instead of trying to calculate this.
The idea is simple. Your seed in the tableau is S. In the round of R (64, 32, 16, 8, 4, or 2) you fence the person whose seed is R+1-S. It only gets complicated when upsets start happening.
In the round of 64 you fence the person with seed 65-S. If you win that bout and it isn't an upset, in the round of 32 you fence the person with seed 33-S. If you do upset someone, you take over their seed, which we'll call S' (equal to 65-S). Then in the 32 you fence the person whose seed is 33-S'.
It's rarely important to figure out this stuff before you see the tableau. Just look at the tableau and you can easily see who you are likely to fence, without having to do any math.
You should now understand how I approach estimating your result at a WC. Go back to the "What Place Can I Expect To Take?" page and go from there.