The two major divisions of theater are comedy and drama. Decide on a tone for your skit based on one of these two categories. Determine a theme for your skit. Identify the skit's intended subject and the point you are trying to make. If this a skit for a class assignment, get to the root of the assignment and build the skit around it. For example, if your class is learning about social interaction in group settings, plan a skit that takes place on a sports field.
Skits generally only have a few characters. You can write a skit that has as few as two characters or even one, if that actor has some especially compelling dialogue and props to work with. Assigning character roles to appropriate actors is also key. Choose roles that suit your available actors well and make sure to choose actors who can effectively inhabit their roles. For example, if your best actor tends to have a quick wit and generous smile, give her a hilarious, pithy character to play.
Good skits have at least one prop, so consider what you have available to you, what you could acquire easily and what you could make yourself. Props not only help the audience visualize your points but also lend more power to the actors' words. Even if the props don't look real, they will assist the audience's imagination in entering the world of your skit. Also include simple backdrops, sound effects or music, if possible, to set the scene for your audience without lengthy explanation.
When writing the skit, keep lines simple and easy to memorize. Only leave parts open for ad-libbing if your actors are highly experienced in this art. Inexperienced actors often freeze or lose the conversational thread if they are not completely confident ad-libbers. Make sure not to give too many lines to one actor. This leaves the other actor(s) on stage standing around with not much to do or say while another character pontificates. Include actions and nonverbal expressions in your script, so the actors know when to move and when and utilize their props. This also makes rehearsal times more-efficient and effective.
By definition, a skit is short. Your instructor or director may indicate how long your skit should be. If not, aim for something between two and five minutes. This not only ensures the audience will stay engaged, but also makes it easier for the actors to remember their actions and lines easily. The most important thing is to include all the basic plot points: rising action, climax, falling action and conclusion. As long as your skit establishes the scene, introduces the problem, solves the problem and provides a satisfying ending, you have a complete dramatic skit, even if it all happens in 120 seconds.