Seller said yes on the phone, now what? I have no idea what paperwork goes where

The order is: written purchase agreement signed by you and the seller, then you open escrow by sending that agreement to a title company or closing attorney, depending on which your state uses. Title runs a search to see who legally owns it and what liens sit against it. Then you either close with your own money or a lender, or you assign the contract, meaning you sell your right to buy at that price to another investor for a fee.

On whose contract to use, both bits of advice are pointing at different problems. A title company's standard form is neutral and unlikely to have a defect that blows up the closing. A contract you bring will usually contain an inspection period and assignment language you care about. Plenty of people use a state association form and add the clauses they need. Have a real estate attorney in your state read whatever form you plan to use once, before you use it twenty times, because assignment rules and what you're allowed to advertise vary by state and some states have licensing triggers around this.

Earnest money is a deposit showing you're committed. Small deposits are common and legal in most places, and a seller can absolutely read $10 as a sign you won't perform.

The part that sinks first deals more often than paperwork: your number. You said it seems workable. Before you sign, get a real repair estimate and three or four actual sold comps within about a mile, ideally from an agent who'll pull them for you. A signed contract on a bad price just means you now have to back out.