It is very simple, and works very well.
With the arc transition and your instrument approach selected and executed. Then - Line select the next waypoint along on the arc to the top of the legs page, then select course intercept on LSK-6R (should be 18.0 ARC L or similar) and execute. When you are finished with radar vectors check that the white track line crosses the ARC prior to the arc waypoint and select LNAV.
It will then draw a dashed line with a turn onto the arc. It struggles a bit if faster than 220 knots on the classic. If you select HDG SEL again the dashed line disappears, you can then turn the aircraft to a different intercept and re select LNAV and it will draw the dashed line again.
You can intercept from almost any angle and from inside or outside the displayed ARC just so long as the track line crosses the displayed ARC.
Have fun as 10.7 works great. (at least is does on our Classic)