Once you've completed the assignment and saved it somewhere on
your computer, then from the main
Blackboard menu, click on "Assignments", which should bring up a page
looking something like this [these screenshots show you the pre-2010
version of Blackboard, but for the current version the lay-out is the
same]:
Next click on the appropriate assignment, which brings up another page like this:
Browse to find the file you've just saved, then once the full
pathname is in the "attach local file" dialogue box, click the "submit"
button in the lower right corner. That should do it! (Do
not paste your work into the "comments" box -- that won't show up as a
completed assignment.)