This is the part where you double check everything before rendering your scene. If there's some things you can't fix or want to add, don't worry. You can fix most of those things in your favorite image editing program. Now it's time to add all the sweet things like Ambient Occlusion and Depth of Field!
Right-click in your viewport and select "Render Settings".
In posters, you won't need motion blur most of the times, so uncheck that. These are my current settings:
Crank up the samples for the Depth of Field to 1024 and play around with the camera settings until you get some nice ambient occlusion. If you want the background to be blurred (camera aperture), right-click on your viewport and select "Display" -> "Show focal plane", and play around with the camera's focal distance and aperture. After that, press F2 (clip editor) and wait until the samples are processed to preview your poster. This might take a while depending on your graphics card.
When you're happy with the result, go ahead and render it by going to File -> Export -> Poster
Make it a 1920 x 1080 image and save it as PNG for the best quality. If your monitor is smaller than 1080p, save your project and close SFM. Right-click on SFM in your library and select Properties, then Set Launch Options and add the following:-sfm_guardband 0 -sfm_resolution 1080
Now load up your project again and ignore the warning that pops up (if it does) and render the poster!