So i want to start speed running this game but i have no idea how to create my own offline match

Right click on the game in steam, select properties, select set launch options under general, and type -scs into the launch options. The option to create server will appear under the servers tab. There is also more information about the game in the Discord and people are much faster to answer your questions there. Alternatively, if you press ` to open the console, you can type "host nondedicated" which should give the prompt to start a local server.

ok thx