- Support the boom with a brace when taking off the forestay.
- Ease tension on the opposite one you loosen. If you pop the forestay, loosen the backstay two turns (the exact amount depends) You don't want them wobbly, but not stiff either.
- Pop one at a time.
- Most people use halyards as temporary stays, but it's generally not needed.
- It's fine to be spooked out about the process.
- You can use your topping lift
, depending on how it's fit, for a back stay (and shroud).
I doubt you would ever do this, but just to make it abundantly clear to anyone who finds this thread in the future: do not secure any part of the rigging including temporary stays to the dock. I saw it done; fortunately it just ripped the block off the mast but it was still gross and needed to get repaired by a metal fabricator.