Thank you for your letter. Here are some general comic strip cartooning tips that I've tried to make helpful for people at all levels of advancement. I hope these will apply to your work.
1. Materials are not important, so long as your work reproduces and reduces clearly. It's what you do WITH the materials that counts.
2. I think characters are more important than jokes. Any cartoonist ought to be able to come up with funny gags, but the best strips have rounded, complex characters that readers can care about. Cartoon characters should be more than standing props to deliver jokes.
3. Don't imitate other strips. Editors are looking for something new and original.
4. Most importantly, have fun with your work, and practice writing and drawing all you can.
(Signed, 'WB Watterson')