Write a synopsis of your story, followed by an outline defining the main plot points. Think visually and make notes about any funny actions you will be able to work in. Since you're writing a silent funny short story, you won't be using any dialogue to get your jokes across. Everything you write will need to be visually funny to your readers.
Use a humorous metaphors to strengthen your text. An example of this might be:
Gerald woke up to the sound of an alarm clock. He reached over and slapped it off, then sat up and swung his feet to the floor, standing up a little too quickly--he was still catching angels from the night before.
The phrase "catching angels" is a metaphor that indicates Gerald is still a little drunk, or high from alcohol. Catchy metaphors can help bring humor out in your silent comedy.
Get inside your characters' heads. You can't write dialogue, but they can think on the page. You don't want to do this too obviously, by adding a dialogue attribution to every line your characters think, so you need to work the thoughts into the narration itself.
He sat on the park bench and his cheeks tightened--not the cheeks on his face. Cold, was what it was, and not just a little cold either. He felt as if he'd just sat on Frosty. He started to stand and came to an abrupt halt as his bottom clung to the icy bench; he realized the pants he'd worn today had been the ones he'd ripped the seat out of the day before ... now his bottom was in a bind.
In the above example, there are no thought attributions, but the point of view is clearly from the character who has just gotten his bottom stuck to an icy park bench.
Develop the scenes you write. Don't just go with the first draft. What would make the above scene funnier? Bring someone else along who might stop to chat with Gerald, who will then need to pretend everything is OK. Maybe someone heavyset comes along and sits on the bench beside him, taking up so much space that Gerald has to continue moving over, in spite of the fact that his bottom is stuck to the park bench. He doesn't want to admit this, so he tries his best to give the heavyset person room without giving away his predicament. There are many ways to add visual humor to a story.
Watch people and take notes. You may not use the notes right away, but there will come a time when you're dry and you can refer to your notebook for something to prompt you. People do funny things in public without ever saying a word. If you watch enough of them, you can fill a notebook with humorous, silent comedy.