Now I prefer to stretch an idea as far as it will go, then a little further. If you can learn to write a blistering 60-second skit with four laughs, a beginning, a middle and an end, then everything else will be easy.
While Weekending is no more, there are plenty of sketch shows on radio and TV that invite outside contributions. If there are lots of writers' names in the credits, write a couple of sketches in an appropriate style (even if it's not your particular sense of humour), send them to the producer and you will probably get feedback.
Or you can set up your own sketch group and take a show to the Edinburgh Fringe or film it for YouTube. Try to make your own material as original as possible. When Stewart Lee and I began writing together at university, we set rules about things we wouldn't write about: celebrities, parodies of TV shows, political satire, all of which were in vogue. By limiting ourselves we came up with a lot of unusual ideas and created our own voice.