You probably want to look at purchasing them before 7 months of your trip as they can only be purchased through MS 1 hour after online booking has started.
I use David’s calculator and look for one day near the start of my use year that is exactly 24 points (or a little more if I have some points) that most likely would be available such as preferred RIV studio, CFW cabin, PVB studio, VGF resort studio. After finding that day I call at 7 months from that day and purchase the OTUP for that reservation.
Afterwards I cancel it and as
@DanCali mentioned it 2 12 point contracts will appear in your dashboard to select when booking at 8 am at 7 months along with your BLT direct points and other resale if you instead want to try for PIT instead of RIV.
Even though the contracts show in your dashboard they DO NOT give you the 7-11 month booking advantage and can still only be used at the 7 month window.