The api-guide.oforce.com will give you specifics for accepted payloads on endpoints, but the process breaks down like this:

POST to /settlements to open a settlement. Get settlement_id from response.
POST to /settlements/:settlement_id/commissions or /settlements/:settlement_id/deductions as needed
PUT to /settlements/:settlement_id with status: 2. This will set it to “Ready for processing” which will consume your settlement into the queue for processing.

Additionally, commissions and deductions are only accepted for contractors already in the system (through the enrollment process). You can get a list of all contractors from the /contractors endpoint.