I've been slowly on boarding some friends onto a personal #pixelfed instance, but it's been really disappointing how difficult it is to login to a custom server with the official app.
1. Don't click the login button! Select settings in the corner
2. Turn on "Allow custom server"
3. Open list of servers
4. Scroll to the bottom and select "other"
5. Now tap login
6. Ignore the presented list and search, click "login with custom server"
7. Enter in your URL and click login

The select "other" custom server screen. There is a button titled "Tap here to login with a custom server" followed by a search able list of servers with their banners next to them.

A list of suggested URLs that Pixelfed shows you when you tap the server drop down. All the way at the bottom is the option "other".

Pixelfed settings screen just off the welcome screen with API permissions toggles and a toggle for allowing custom servers.