First thing you wanna do is render your poster. thats done by going into file, export, movie, select the option "image sequence" for Export. then choose format (i picked PNG). Choose your resolution. and for duration chooses "custom" next to it are numbers. make sure it says "0" and "1" (there should be a option for frames or seconds but it doesn't matter which.

Go to file and select "community" and select "publish image to steam" it will give you some fairly simple fields to fill out. on the top right is image. all you have to do is find your image in the sfm folder. give it a title, a description. visibility (you can change it to how you like) game which its from, and type (by default it should say poster. and hit "publish image"

I see alot of blokes that work on this say "use the snip tool" i find this easy to use than snip tool. also share this and leave good feedback. also this is my first sfm community guide. thanks for reading