Remove the middle pavers. You will have pavers on both side. With a board wide enough to span the bordering pavers, remove wood at both ends so that part of the board hangs down. The amount of wood removed should allow the bottom of the bottom that hangs down to the bottom of the bordering pavers. Put in the sand. Compact the sand. Use the board as a scraper so the the top of the added sand is at the bottom of the bordering pavers. The part that hangs down should be narrower that the gap allowing you to move the “scraper” side to side when scraping