Simply line up the projected image of the sun with the centre circle. The 45° screen makes viewing easy.
Lower image shows solar finder fitted to a standard 50mm finder mount (not included in price).
These images show a simulation of the sun shining on the finder, centred in the upper two and tracking to the centre in the lower.
The keen-eyed will notice that this solar finder is fitted the opposite way to normal, this is because I have to slide in the holder reversed
on my Skywatcher 200, having mounted my Telrad finder very close to the mount on the OTA.