Beginning of creating a multiplayer game

How do I plan to create a game? Is it on paper? do I list all the assets I have to do? characters action?

I want to create a third person action multiplayer game…any idea which should I tackle first?
I have strong conceptualization

http://img13.deviantart.net/ddc2/i/2016/347/9/d/allaversus_by_alexandersinnelius-dar3qu6.png

http://img15.deviantart.net/36fb/i/2017/047/0/2/lalalala_by_alexandersinnelius-daz9l8l.png

But I guess I want solid planning and planned ideas of the list of things I have to designs…
Ideas? I plan to keep this updated as often.

i would start with the basics and get that working. then get the multiplayer set up and modify the basics to work with multiplayer, then the landscape and detailing. the UI doesnt really mater yet i would do it somewhere after creating the multiplayer so that you can immediately integrate that into the main menu or other UI you use for joining/creating multiplayer (servers?, [if you use servers])