Bed and level the first block
Set an end block or a half block in the mortar bed, centered between the chalk lines and exactly aligned with the end line.
(You may need to scrape away mortar to see the line.)
Press it into place so that the mortar is about 3⁄8 inch thick. Check the block for level in both directions and check the face of the end for plumb. Scrape away excess mortar and use it for the next block.