this was pretty painful to get to work but heres what i did in the end:

you can clone the feed-generator example repo from here GitHub - bluesky-social/feed-generator: ATProto Feed Generator Starter Kit

here is my edited version, which currently shows users with a custom (non .bsky.social) domain as their username. its probably a bit scuffed.
GitHub - iamasink/feed-generator: ATProto Feed Generator Starter Kit modified

how to :D

kinda tutorial but also idk what im doing, so basically just what i tried that worked:

first, make sure everything works locally, the app seems to cache the feed so you may need to create a new one (maybe with a different name?) if you mess up and it errors out.
example .env

# Whichever port you want to run this on 
FEEDGEN_PORT=3000
 
# Change this to use a different bind address
FEEDGEN_LISTENHOST="localhost"
 
# Set to something like db.sqlite to store persistently
FEEDGEN_SQLITE_LOCATION=":memory:"
 
# Don't change unless you're working in a different environment than the primary Bluesky network
FEEDGEN_SUBSCRIPTION_ENDPOINT="wss://bsky.network"
 
# Set this to the hostname that you intend to run the service at
FEEDGEN_HOSTNAME="testblueskyfeed.iamas.ink"
# change !
 
# Set this to the DID of the account you'll use to publish the feed
# You can find your accounts DID by going to
# https://bsky.social/xrpc/com.atproto.identity.resolveHandle?handle=${YOUR_HANDLE}
FEEDGEN_PUBLISHER_DID="did:plc:txdeqi6p3r4hdz4ppa7bj5zh"
# change !
 
 
# Only use this if you want a service did different from did:web
# FEEDGEN_SERVICE_DID="did:plc:txdeqi6p3r4hdz4ppa7bj5zh"
 
# Delay between reconnect attempts to the firehose subscription endpoint (in milliseconds)
FEEDGEN_SUBSCRIPTION_RECONNECT_DELAY=3000

change the hostname and publisher_did to your hostname where you host the feed and profile did
you must also change the feed name in the algos\whatever.ts. this is the shortname¹, you must set it in the script later

you can visit the following url to check if your feed is being populated

http://localhost:3000/xrpc/app.bsky.feed.getFeedSkeleton?feed=at://did:plc:<your did*>/app.bsky.feed.generator/test

* as specified in .env

once it's working locally, you need to make the feed available on https on the internet.
for example, I'm behind caddy and cloudflare.
in the caddyfile:

https://testblueskyfeed.iamas.ink {
	# email for tls
	email example@example.org
	reverse_proxy localhost:3000
}

ensure your firewalls allow port 3000 (or whatever you choose) to access it, then check your public domain/subdomain at a similar url to earlier:
(no port)

http://myfeed.example.org/xrpc/app.bsky.feed.getFeedSkeleton?feed=at://did:plc:<your profile did*>/app.bsky.feed.generator/test

once you are sure it is working, and can see posts being populated in the feed on your public url, create the feed on bluesky using the publishFeedGen script.
notably, the short name / record name must be the same as in the algos\*whatever*.ts file.

errors i got

Hmm, the feed server appears to be misconfigured. Please let the feed owner know about this issue.
Message from server: invalid feed generator service details in did document:did:plc:txdeqi6p3r4hdz4ppa7bj5zh

im pretty sure this error was because i tried using the FEEDGEN_SERVICE_DID and setting it to my account id?

Hmm, the feed server appears to be misconfigured. Please let the feed owner know about this issue.
Message from server: invalid feed generator service

probably same as last error?

Hmm, some kind of issue occurred when contacting the feed server. Please let the feed owner know about this issue.
Message from server: Unsupported algorithm

from having a different shortname for the published feed as to the shortname in the argos file

or if while testing the manual url, you get this
{"error":"UnsupportedAlgorithm","message":"Unsupported algorithm"}
its from having the wrong DID or feed url, basically any part past feed= being wrong causes this i think.

Hmm, some kind of issue occurred when contacting the feed server. Please let the feed owner know about this issue.
Message from server: XRPCNotSupported

probably you pointed to the wrong domain, or are hosting the wrong thing on the port you chose.
happens to me if i run a nextjs dev server on port 3000 and visit the feed.

actually making a feed

i wanted to see if it was possible to create a feed that was all custom domain handles..
unfortunately the firehose doesn't return the handle, only the DID so i have to resolve the handle for every single post..
so i'll just keep a cache of users that have been resolved so i can slightly reduce the load, surely it'll be fine...
it hasn't caused any issues so far but i imagine if i run it for longer (days+) then it will have some issues.