Put the heated bed assembly in its place and slightly push the rods to holders.

Pass the GT2 belt from idler pulley and motor pulley. Teeth should be inside.

Insert the belt ends to the toothed gaps in the Y Belt holder. Try to make it as tight as possible but to not force to make it as tight as it should be. Because you have the belt tensioner for this purpose.

Tighten the belt with the tensioner. Turn the two screws in order, one full turn each time. As a tension, the belt should "twang" when you flick with your finger.