So I know this isnt technically about pickup, but it's related so hopefully the mods allow it.

First of all let me say; seducing a friend is virtually the same as seducing a date in the final stages. Build intimacy, Escalate with consent. The difference is you haven't signalled intent beforehand. So the intimacy you're building will default to platonic intimacy rather than romantic intimacy. For some that will be a big obstacle. But not for everyone. If you can also build sexual intimacy and turn them on enough then you can still potentially escalate.

So that being said there are 3 things which I feel can support that escalation. And they all have to do with how your friends perceive you and your reputation. These are just my own observations from my own experiences so feel free to add some points or alternatives in the comments.

  1. Prowess. Do you have the reputation for being good in bed? Do you actively cultivate this with sex talk? If you can cultivate this reputation your friends will be more open to sleeping with you. They'll know it's gonna be a good time, not a waste of time.
  2. Respect. Do you have a reputation for respecting your sexual partners? If your friends know that you don't talk shit about past partners, don't gossip about them, don't engage in 'locker room talk' then they'll be more likely to sleep with you. They need to know their reputation is in good hands.
  3. Attitude. Do you treat sex as an adventure? Are you gonna fall in love with your partners? Does it always have to be about romantic intimacy? If your friends think you'll fall in love with them they wont touch you. But if they know that sex can just be sex they'll be more open to sleeping with you. Cultivate this reputation

So to summarize you want your friend, in the moments leading up to escalation to think 'X is really turning me on. They're good in bed. They won't fall in love if we hook up. I know I won't have to worry about gossip. So why not? It won't change anything, we can just start being fuckbuddies too.'

Other than that it's the same as any other escalation. But those 3 points can, in my experience, really help you to set up some great FWB situations. Critiques and additions welcome in the comments.