Step 1: First row (WS): With yarn B, purl the first stitch.
Step 2: Bringing A over B, purl the second stitch.
Step 3: Always brining the alternate yarn over the yarn just used, continue purling in this manner to end of row.
Step 4: Next row (RS): Brining B under A, purl the first stitch. Always bringing the alternate yarn under the yarn last used, continue purling in this manner to the end of the row, ending in B.
Check out these Pom projects which use Latvian braids: