1) First step of course is to upload the sketches! You can either upload them as you go along and are working on the sketch or upload them all at once when you're ready to show off the final sketch. Either way you want to upload them in the order of creation - like step 1, then step 2, then step 3, etc. (The order you upload them in determines the order they are displayed as WIP thumbnails.)
2) After uploading sketches you want to link them up so that when someone views the final finished sketch they see all the work in progress sketches as little thumbnails underneath it. To do this go through each of the work in progress versions you uploaded, view it and click "edit this sketch", scroll to the bottom and you'll see thumbnails of all the sketches you've uploaded recently - tap on the one that's the final version of your sketch and hit update.
I hope that's understandable! Reply here if you have any WIP questions!