Think of people in your life that make you laugh. People are funny for two reasons.The first is that the person might be genuinely funny, by cracking jokes or acting in funny, zany ways. But surprisingly, these characters don't work in a comedic script because they seem like stand-up comedians, instead of what they should be -- real characters in real situations. The second type of person is funny without realizing he is funny. Think of your neighbor who is obsessed with decorating his house with as many Christmas lights as he can, or the dry cleaning lady with the thick glasses and the high-pitched voice. Extreme characters add comedy to any script. Decide first on your protagonist, and which type of character he is. He might be the straight man in a world of crazy characters, or maybe he's the craziest character and the rest of the world thinks he's nuts. The characters are what make your script funny.
Write a funny story. This sounds like obvious advice, but it means don't simply write a lot of jokes. Write a story with plot points that are genuinely amusing, and that organically appear through the interaction of your characters. Your script could tell a tale of a madcap adventure through a forest, or a tale of a mundane office complex on the day something crazy happens. Unlike stand-up comedy, jokes in your script must have something to do with the story. The funniest parts in comedy scripts are completely based in the story you're telling. Hold off on the jokes, and construct situational comedy with the characters and their surroundings.
The basic construction for a comedy script is: Act 1, the audience meets the characters and the problem is introduced. In Act 2, the problem becomes more complicated and the main character struggles to right the situation into which he has gotten himself. Then in Act 3, the problem is solved, either by the main character or by another series of events. Shakespeare knew that when writing a comedy, he had to tie up all loose ends in the end. Comedies resolve happily at the end, unless you plan on creating a series of scripts, in which case you can create a cliffhanger at the end.
The dialogue in your script should sound natural and non-threatening to the less educated audience members. Though William Wycherly wrote high-brow Restoration comedies in his time, comedy has changed since then. The dialogue should be clear and make sense to the audience. Read your script aloud, or have actors read it for you. Use your judgment. If it doesn't sound natural or funny to you, odds are it won't sound natural or funny to an audience.
Gather a group of actors to stage or present your play in a reading. Invite the general public. Invite your close friends only if you must. Their reaction to your work won't be as genuine as that of the general public. You may have included some inside jokes in your play, which your friends will realize and find hilarious. The general public might think that part of the play isn't funny at all, because they don't know your inside jokes. Take notes during the reading about what worked and what didn't. Then go back to fine-tune your script.