To make mortar, mix sand and cement, sometimes with the addition of lime, using the proportions below for specific jobs. The lime reduces the risk of the mortar cracking as it dries, and makes the mix easier to apply.
Never make up a mortar mix that will dry out harder than the bricks themselves. If the mortar is softer than the bricks, any cracks caused by settlement will follow the mortar lines and not affect the bricks. A mix of one part of cement to five or six parts of sand is usually satisfactory – but check this. For small repair jobs, use one part of cement to three parts of sand.