Listing on Satflow programatically involves a few things:

Generating Listing PSBTs using /intent/sell
Signing all Listing PSBTs using local wallet software
Submitting all Signed PSBTs to /list

Generating Listing PSBTs

You need a few things to generate a listing PSBT

collection_slug is entirely optional, no need to provide this.
Required fields:
- inscription_id or runes_output - one of these is required to indicate which item is being listed
- ord_address This is the owner address of the specified item
- receive_address This is the address for receiving payment from the sale
- price - The listing price, in satoshis
- tap_key - The public key of ord_address, usually 66 characters long, and is hexadecimal format

Now you need to POST /intent/sell with these included in the body.

You will receive unsignedListingPSBTBase64 and secureListingPSBTs. Since you're listing one item, you will receive a total of 2 PSBTs to sign.

Signing all Listing PSBTs using local wallet software

One of these is for the insecure (snipable) with sighash SINGLE | ANYONECANPAY. The other is for secure listing (where sniping is not possible) and this is sighash ALL | ANYONECANPAY.

You will need to sign these two PSBTs using a wallet of your choice. The insecure listing PSBT has only one input - you can just sign that. The secure listing PSBT provides a indicesToSign field.

Make sure you are signing with the correct sighash at the correct indices.

Submitting all Signed PSBTs to/list

Now that you have signed both PSBTs, you will need to reuse the original parameters which generated the listing (required fields in section 1 of this guide).

In addition to those parameters you need

signed_listing_psbt in base64 or hex
signed_secure_listing_psbt in base64 or hex
unsigned_listing_psbt is optional!

POST /sell with these parameters and you've successfully listed!

Notes

To edit a listing you are not required to cancel a listing first, just repeat the above steps to replace your listing.