Not my video but here is the solution if you are struggling, I put it in the spoiler tag so it loads faster:
This can be achieved as soon as you enter Animus Island. Or leave the animus from the menu to return to the animus island. Instead of running toward the gate the game tells you to, turn around and go into the portal that is right to you.
You can complete the Desmond sequences in any order as it appears they are all accessible from the start. Recommended that you complete Desmond Sequences 1, 2,3 and 4 first as these have a story arch to them.
Also here are the other sequences:
1:
2:
3:
4: