Disclaimer: I’d like to make sure it’s understood that these are in no way to be taken as qualified instruction. I suppose that technically I should classify these as ‘reminders’, but it is possible to learn from them… Just make sure you check with someone that actually knows what they’re doing before you try and do them halfway up a cliff.
If you want to tie a figure of 8 in the middle of a rope, it’s pretty much the same as how you tie a normal one, but with a bight, rather than just a tail.
Start by passing the end of the bight over the two lines (any sailors will probably get annoyed with me for that misuse of terminology…).
Pass the end of the bight back underneath them again.