Overview
In Settings there are 2 options relating to network multiplayer. The first; Name this is pretty self explanitory but changing this changes your name in network multiplayer games. Very useful.
The next, As Host has two options, the first is Play Hosted 1v1s and the second is Spectate Hosted 1v1s
If you select the first option your netplay ID is set to be 0 with everyone else starting at 1. If you select the second option your netplay ID is set to be the highest (if 2 clients you’ll have a netID of 2, 3 clients; ID of 3)
This is important because with 1v1s net clients with a netID of 2 or 3are set to spectate the 1v1 not play the 1v1. The ones playing have netIDs of 0 and 1. This lets you host the game - they still connect to you - but you spectate it. Useful to stream, record, moderate or whatever for 1v1s.

To play Network Multiplayer on the main menu select Network
If you are hosting a game (players connect to you) then you’ll click host
AND WAIT ON GAME SELECT SCREEN UNTIL EVERYONE HAS CONNECTED
If you are joining a host then enter the host’s IP and click connect. If it stops responding and looks like its going to crash it means it could not connect to the host.
There is always one host and 1-3 clients.
Localhost
If you want to have multiple windows of deathmatch running and play against each other have one window host, and the other(s) select connect. Leave the ip blank or put localhost and it should connect just fine. (make sure to set Player 1 to use a controller on one of the instances)

Local Network
One player host, the others connect. To get your IP open command prompt and type ipconfig, it’ll show you your ipv4 address. Use that.

Online Multiplayer
While not “officially supported” you CAN set deathmatch up to do this. All you have to do is have your host port forward (tcp port 8594) and give you their public ip. This should work just be aware bad connections and lag can make the game unplayable. If you don’t know how to port forward just google it, its slightly different for different routers.

Last Note:
Network multiplayer does have more bugs and way more crashes than local multiplayer. It is likely to crash one player and not tell anyone else. Just keep this in mind while playing.