To submit your entry, visit the challenge webpage and log into your account if necessary.
Next, scroll down a bit until you see the orange "Submit Entry" button on the right-hand side of the page - this will bring you the the "Create Submission" page. At this point, your entry is comprised of several questions/text fields - one of which is giving your entry a title/name. Required fields are labeled with an asterisk. If you scroll down the submission page, you will see a section labeled "Submission Document." Within this section are three "Browse..." buttons - one for each of the three support categories. (If you are using an Apple system, the buttons are labeled "Choose File".) Click the button(s) associated with the appropriate category and then navigate to your submission pdf.
At any point while you are working on your submission, you can save a draft of your entry form by scrolling down to the bottom of the form, and clicking on the blue button labelled "Save Entry." You do not have to complete all required fields in order to save a draft of your entry.
When you want to return to continue working on your form, go to the competition web page again, and click on the blue button labelled with the name you chose. That will bring you to your entry page - just click on the white "Edit" button to enter edit mode again.
Once you are done with all of your edits and are ready to actually submit your form for judging, save your work if you have made any changes/updates (using the blue "Save Entry" button,) and then click on the red "Submit final entry" button at the top of your form to the right of the "Edit" button.
If, after submitting your final entry you decide that you want to update it before the submission deadline, just go to the competition web page again, and click on the blue button labelled with the name you chose, and click on the "Return to editing" button (I am not sure what color it is) and you will be able to edit your entry. Just be sure to save your work, and you will need to click "Submit final entry" again when you are ready.
This may have been more information than you wanted, but someone else may be helped.