Since you have access to the left-side balance shaft from the engine bottom, you might do well to put the B belt cog in place and tighten it. Holding the balance shaft from below allows you to tighten the shaft’s nut without clamping (and possibly damaging) its gear teeth. Be sure to put the flange back properly, and I remind you of the “photographic” image left on the flange face. The crankshaft flange is important, as it keeps the two belts in their own corrals. (Check your manual for an exploded view of the parts to assure you have them in the right order). So we need to put the crankshaft
B belt cog on, and there you pay attention to the flange timing mark positioning...but it can only go on one way, so have some confidence. Set the balance shaft gear to its timing position, get the belt tensioner on, and the belt. Get the belt teeth into the crankshaft gear teeth, and pull the belt so there is tension going up from the right-hand side to the shaft gear. Watch that sneaky crankshaft gear so it doesn't move from your pulling on it. Holding the shaft gear, engage the teeth, then loop the slack part of the belt above the tensioner. Tighten the tensioner according to the manual’s instructions. Make sure the 2 timing marks...the shaft gear and the crankshaft...are still perfectly aligned after the tensioner is tightened. If not, loosen the tensioner and work them until they are. The tensioner is pressed against the belt "finger tight", according to the manual. It does not need a lot of tension...it's only turning the balance shaft...and too tight of a tension will cause Feng Shui issues with your wife and second cousin, according to Lilian Too, but only in the Year Of The Tiger.
Check your manual's photos for timing positions, and I have also provided one for you below.
Once the B belt is installed, we’re ready for rock ‘n roll.