go to garrys mod in your library and click on the cogwheel. go to manage, then browse local files. go to the bin folder, and scroll down to hammer.

go to the valve developer wiki and follow this guide: https://developer.valvesoftware.com/wiki/Your_First_Map

take the map file and download GMod pre-publisher: https://github.com/WilliamVenner/gmpublisher
then, insert your map file and gmad file. your gmad file can be found in bin aswell. fill out all the other info and add a thumbnail if you like. once finished, create the gma file.

take your gma file and download Crowbar: https://psteamcommunity.yuanyoumao.com/groups/CrowbarTool and follow the installation guide. when finished installing, configure the game to be garrys mod, then fill out all the info and upload your thumbnail again, then hit upload. once finished uploading, you can go to the workshop page and make further updates. i hope this guide helped!