try and keep it centered, but doesn't have to be perfect. Use vermiculite and fill between tile and block, every foot or so put a few dabs of mortar around the corners of the tile to keep it from shifting, not much, about a tablespoon full, let it set on the vermiculite that you've already filled the chase with.
It's best if the joints dont' line up but not essential.
TIP: the ends of the tile are *NEVER* cut square, so take a square and mark the tile you've just laid as LL on the longest side and L on the next longest side, then mark the tile you are about to set the same, put the LL and L sides opposite on the joint so that the tile stack stays straighter - you'll spend a LOT less time shimming joints just to keep it straight and your joints will be tighter. If you have any tiles that are cut almost square, mark those as such and set them aside, also mark the ones with two parallel sides and one long side with an L and set those in a separate pile.