So here's what my finished product looked like:
These seem simple enough, but I took pictures while making them, so if you want a step-by-step, here goes :)
First, you'll need some fake flowers, mason jars, ribbon, and some sort of pretty eye-catching dealio to put on the ribbon. The easiest way to do flowers is to buy a bunch of fake floral bushes and cut all the individual pieces off (it's cheaper and you buy less stuff).
|Cutting off individual flower stems|
|After stems have been cut off|
And now we glue that to the mason jar.
This is also done with hot glue. (It does come off, but it takes some doing. In case your mom wanted her mason jars back) Notice that ragged edge where the ribbon comes together? Ick. That's where the pretty eye-catching dealio comes in. It could be a button, or whatever else you have laying around your craft room that's hopefully flat and appropriately proportioned for this purpose.
Now we arrange the flowers. Personally, I found that it's easiest to deal with them if you stuff a bunch of leaves in the bottom, to hold things in place.
Place your stems from earlier in the jar. You may need to cut stems shorter to account for the height of the jar. Mess around with it till it looks nice.
And ta da, you have cute floral centerpieces for a grad party or whatever else you might need them for! :D