I don't have any handy "start to finish" guides for this. I suppose I should start a blog, or something :)

Basic steps:

1. get a phone that can run PostmarketOS - https://wiki.postmarketos.org/wiki/Devices

it does not need to run it perfectly. Camera, cell modem, even screen are completely optional. It must charge and do WiFi, only.
2. flash PostmarketOS on the phone. Get SSH access - if the screen doesn't work, use USB ethernet emulation (during pmbootstrap init, choose USB mode = "developer"), log in, then nmtui to enter your WiFi password.
3. optionally, reduce maximum battery charge/voltage, to prolong battery health. https://wiki.postmarketos.org/wiki/Battery_Voltage_Reduction
4. hooray, now you have a server. Proceed setting up as if it were a VPS. It's a variant of Alpine Linux - any Alpine guide will work fine, or you could use Docker or Podman.

Now this is something I haven't done, but you could - hook it up to a solar panel. Rely on solar during the day, and on the phone battery until it lasts. Write a shell script / cronjob / systemd unit to suspend the phone when battery level gets low. It'll get a wakeup event from hardware, when solar comes back (hopefully).

Offsite monitoring. It runs Prometheus, Grafana, etc, and watches the rest of my selfhostables. Sends me alerts when stuff goes down.