After crossing the main trail Dragalevtsi - Bai Kristo (E4), continue west crossing two wooden bridges. After the second, stay to the right on the wide, slightly uphill path that brings you near to Dragalevtsi Monastary.
You'll arrive at a hairpin turn in a road; cross the road and head down the hill maybe 100m to find the marked trail going left into the woods. Here the trail climbs mostly gradually and crosses a few more small streams.
You'll pass a few moraines for which Vitosha is known as the trail hugs the turns of the mountains. Upon crossing a meadow, the descent starts, first gradually then more steeply, until you arrive at Boyana Lake.