Starting:

Navigate to the "server" page on Aternos, (the "home" page of your Aternos account). Then click start.

Depending on what your server is (vanilla, snapshot, modded, etc.) it may take longer to start up. Once it is done, you will see this:

The timer near the left of the screen shows you how long till the server shuts off (unless someone joins, in which case the timer will disappear).

Joining:

First, start up your server. Then once you are at the screen above, ^ click "connect". It will show something like this:

The IP is the address anyone who has Aternos access to your server can use. The Dyn IP is the address anyone else has to use.

PLEASE NOTE: the Dyn IP changes EVERYTIME your server starts up!!! so make sure to send your friend the new Dyn IP every time!!!

PLEASE ALSO NOTE: The IP does NOT change every time your server starts up, (to make the server owners life easier) so it might be good to give your friends Aternos access to the server so they can

A) Start up the server themselves

B) Use the address that doesn't change!

That's how you (and your friends) connect to an Aternos server!