Always keep a notebook on you. You never know when a funny idea might come to you, so make sure to always have a place to write it down so you don't forget it later. Use a blank notebook with no lines so you can also quickly sketch out ideas or characters if the need arises.
Understand the joke you are telling. The joke is the most important part of the comic strip; it's what people will remember more than any of the illustration involved. Spend much of your time coming up with the joke itself. If it's a three-panel comic strip with a punchline at the end, make sure you have the perfect phrasing for the punchline. If it's a one-panel comic strip with a caption underneath, try a few variations of the caption to see which one works best.
Use simple visuals. If you are using complex visuals and intricate designs, that may confuse the reader as to the joke of the comic strip. Unless complexity is somehow involved in the joke of your strip, maintain sparse designs that get your joke across without the reader getting distracted. Comic strips are meant to be read quickly, almost like you'd be reading a joke from a joke book.
Make a rough copy to show off. Before settling into the final draft stage when images and phrases are locked in, show off a rough copy to gather opinions from other people. The biggest thing to learn in this process is just whether or not your joke is actually getting across, or if the message is muddled. Once you get past that hurdle, you can begin composing the final draft of your comic strip.