You need a construction of 3 wooden slats. 2 vertical and the third in 45 degrees in between (you can change the angle of the third slat, so you can go for the right angle between 40 and 45 degrees)
When shooting, place next to the construction and make sure that the arrow has the same directional position as the third slat.
If you have practiced enough, you wil not need the construction anymore.