Finish a long wall with a closure block
The last block on a long wall, called the closure block, is a bit tricky to install.
If you need to cut a block, cut the next-to-last block and then install a full-sized closure block. Spread a thick layer of mortar onto both ends of the closure block. Align it precisely with the center of the opening and carefully slide it straight down into place. If mortar peels off one of the ends, pick the closure block up, reapply mortar, and try again.