How do I install this flush-mount ceiling light?
First measure this distance on the light base:



Then check this distance on the existing bracket. (These should be the threaded holes in the bracket).



If those two dimensions match then use the screws B to mount the base to the existing bracket into the threaded holes. You should be able to start the screws into the bracket and then slide the light base over the screw heads in the larger part of the keyhole slots in the base. Then turn the base so the narrow part of the slot goes under the screw heads and tighten the screws till the base is secure against the ceiling.

If the existing bracket does not match then do a sanity check of the new bracket of the measurement between two of its matching pairs of threaded holes as for example:



When you find a pair of threaded holes in the new bracket that match the hole spacing in the light base mark the pair that will be used. Remove the old bracket by removing the two screws as noted here:



Install the new bracket in similar position as the old bracket using the existing screws through the slotted holes of the bracket. Center the new bracket over the box. If it is longer the ends of the new bracket may ride out over the box and rest on the drywall surface. Secure the bracket into place. Then start the screws B into the marked holes. Then, as noted before, slide the light base over the screw heads in the larger part of the keyhole slots in the base. Then turn the base slightly so the narrow part of the slot goes under the screw heads and tighten the screws till the base is secure against the ceiling.