Magneticflip,
here is how you can visualize it on the DG/HSI:
consider the center of the instrument with the airplane symbol to be the NDB station, if you work in QDM, the bearings are TO the station, so draw a mental line towards the middle of the instrument.
Lets suppose you are on a QDM of 270, with a heading of approximately 270 (+- wind correction). In this case the current QDM lies from the bottom of the instrument straight up towards the center, pointing towards 270. (draw it on paper if not in the airplane).
So, if you have to intercept at 90 degrees for example QDM300, first visualise where QDM300 is situated, draw a mental line on the DG from 120 to 300 degrees, stopping in the center.
It is evident that the required QDM is to the left and that you have to subtract 90 degrees from the required QDM (300-90=210) to intercept it.
Fly a heading of 210 for some time.
When the ADF needle will be on your right hand wingtip, then you are established on the QDM300.
In practice, you would start to roll out of the 90 degree intercept about 10 degrees before (at 080 degrees, 10 degrees before the right wingtip).
Following the same visualisation method, try to work out how to intercept a QDR?
I hope this helps. The difficulty for a newcomer is that you have to mix different angles: heading, bearing to the station, ADF needle versus aircraft nose.
I recommend a PC IFR flight simulator for properly learning these things. I use Jeppesen Flite-pro for keeping current on procedures. I recently discovered x-plane (
www.x-plane.com) as a very fine simulator with excellent flight dynamics. My favorite is the C119, or the PC-9.
Dirk