Now you should be holding the spindle nice and tight. Take that flathead screwdriver and slide it through the spindle so that it cannot retract. (in this pic I used a drill bit works the same. I have also used a small handled paint brush)
Set the spindle aside in a safe place you will not need it until later.
Remove the Anchor and the Pulley.
now that you have the seat-belt freed from the car roll it up put it in a bag whatever it takes and set it aside. (note all the hardware will be on it. consisting of the ANCHOR, PULLEY, PLASTIC PIN [may be white or black] and the BUCKLE. the buckle stoppers are disposable.)
REPEAT the process for seat-belt number two.
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If you want you can just remove all the assemblies intact. IE with the fabric still rolled on the spindle. From there as long as the spindle is oriented as it would be in the car you can unwind all of the seat-belt. (the reason I did it different above is it is easy to unwind while still on the roll-bar and also I had my new belts ready so it was just a fast swap.) That way you can do all the unwinding and removal of the fabric in your favorite lazy-boy. Just make sure all the bolts and hardware is in a plastic bag or in a place where you will not lose it in the interim.