So first you want to click multiplayer. Then click play with friends. Make a world if you haven't started one. Once the world loads hit escape. You need to share the Staxel net address with your friend. Click the eye to see the address and then click the copy button next to it. Send the code to your friend in steam chat. Your friend needs to click multiplayer and paste that code (CTRL + V) in the server address.

you can do it the opposite way if you want to join their farm. Remember that it will keep the previous code in the server address and the codes change every time you play.

Hope that helps you!