From the "Research" group of equipment, build the "Stir Research Station" on or near the sand using 5 planks and 1 pole. Grab a bucket and use it to take about 20 buckets of sand and put each one into the "Stir Research Station". After the research has completed, a message will be shown about a new blueprint that can be found in the "Misc" group of equipment for the "Crude Mortar Mixer", which then can be made using 5 planks and 1 pole. Mortar can be made in the Mixer using 1 clay, 1 bucket of sand, and 1 bucket of water.