Almost everything needed is available in different parts of the wiki. This will point to each one to get you up and running.
Once the client is setup locally and you can see it internally (example: localhost:5454) and externally (example: http://<your-external-ip>:5454 or https://notifiarr.<your-domain>.com) then you need to tell the site how to talk to it. You can see how to do this here. If it isn't green on the site then the URL is not going to work and needs fixed. This can be many factors but common ones are a bad port forward, a cloudflare block, bad configuration, timeout, etc.
Use curl and test it (the /api/version is important and validates the apikey check from the site to the client)
curl -I -H "x-api-key: \<notifiarr-apikey-here\>" -H "Content-Type: application/json" \<client-connection-here\>/api/version
Example:
curl -I -H "x-api-key: 123456-7890-12345-6789-0123456789" -H "Content-Type: application/json" https://notifiarr.my-domain-dns.com/api/version
If this does not return a 200 response with a JSON payload, it isn't right. Check your port forwarding / reverse proxy / cloudflare configuration. You can also check the time it takes to get a response. The client needs to respond within 7 seconds.
curl --write-out '%{time_total}' -I -H "x-api-key: \<notifiarr-apikey-here\>" -H "Content-Type: application/json" \<client-connection-here\>/api/version
Example:
curl --write-out '%{time_total}' -I -H "x-api-key: 123456-7890-12345-6789-0123456789" -H "Content-Type: application/json" https://notifiarr.my-domain-dns.com/api/version
If this takes more than 7 seconds it will not work. You'll need to figure out which app is taking so long to respond or is timing out.
First thing, turn on a couple that you want to use (or all of them at once that you want to use if you want but less is more while learning). Once you have them turned on the page will reload and you'll see a card for each one. If you cant figure out how to turn things on etc, each integration has a wiki page for it (well most do). Click the Wiki link in the integration card to get more information about that specific one.