Mix more sand and cement onto the crushed rock foundation. Screed it using your straight edge, to make it level. If there is some sand mixture close to the pavers that you can’t screed, make it level using your hand. Continue laying more pavers on top of the screeded mixture. If there are a few pavers that are tight to fit, use the rubber mallet to tap them into place. After laying the next rows of pavers, check they are level using the string line. Adjust the height of the pavers to make sure they’re all level.