How can one incorporate good humour in their writing? In other words, how does one write witty jokes?

I don’t recommend writing actual jokes because people have very different senses of humor and what one person considers funny another may consider an insult.
Instead, simply include scenes in your writing that are humorous without any jokes. Little kids are always fun. Same with animals, especially baby animals. And there are other scenes that make readers smile. Just spend some time thinking and more will come to you.