Is there a thick cardboard strip at the edge of that upholstery? If so, lay a piece of plywood across the black frame and kneel on the plywood to provide enough slack in the upholstery. The cardboard strip is supposed to tuck once then slide into the groove in the frame.
What Cush said. Your weight will compress the seat spring a bit to give you slack in the cover. Also, warming it with a hair drier or heat gun helps soften it up a bit.
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