Four steps: add the flatpass bot to your server, connect a Stripe account, pick the Discord role you want to sell, and set a price for it. Stripe's own onboarding is the long part, at about ten minutes.
Stripe verifies every account that takes money. Its onboarding takes about ten minutes and wants photo ID and bank details, so it goes more easily on a laptop than on a phone. The optional Stripe Tax step is free to switch on; “digital goods” is the fitting product type for Discord access.
Leaving halfway costs you nothing. Your Stripe account is created before you ever reach Stripe’s form, so coming back picks up where you left off rather than starting again. If you started on a phone, the connect screen can have the bot send you the link in a Discord DM so you can finish on a laptop.
A free tier needs no Stripe account at all — no product, no price, no card. Installing the bot is enough to get an invite page built from your server’s own name, icon and roles, and members can join a free tier from it. Connect Stripe when you want to charge for something.
One thing to get right first: whether a tier is free is fixed when you create it, so a role you intend to charge for should wait until Stripe is connected.
Press Connect Stripe on your dashboard and finish Stripe's onboarding, about ten minutes; no API key is pasted and you can revoke it from Stripe.
Manage Roles and Create Invite. Never Administrator, and it cannot read your members' messages.
Yes, one per server, and free members never count toward the paid-member cap. Whether a tier is free is fixed when you create it.