WTF is a 'QDM'? Bearing to the station? Bearing from the station?
Are you using a fixed card or RMI?
For an intercept at 90 deg (your specific question but it works for other angles). Bit hard to describe without diagrams...
Step 1. Work out where you are. It's easier using an RMI but can be done using a fixed card + the DI.
For all my references to the RMI (if you're using an RBI + DI) you should mentally transpose the position of the needle onto the DI. Count the number of divisions (the 10 deg & 5 deg marks on the compass rose) the head or tail of the needle is L or R of the nose/tail. Note the same number of divisions in the same direction on the DI. That's What the RMI is doing for you. Use your pencil by moving it from the RBI to the DI if you have trouble imagining it.
On the RMI (or RBI transposed to the DI) the centre of the insrument represents the navaid. The tip of the tail of the needle represents the a/c. A/c heading is always oriented so that it points 'up' the instrument panel.
Whatever bearing is under the tail of the needle is the a/c's position relative to the aid. You will also be able to see if the navaid is behind or in front of you as well as to the L or R.
2. Note the bearing FROM the station you wish to intercept. Mentally picture this line on the RMI (or DI). This it the track you wish to intercept. If it's a bearing TO the station then after you find this postion, look at the opposite side of the RMI ie the reciprocal.
3. Compare where you are (the tail of the needle) to the the desired track to intercept. If the desired track is somewhere to the R, turn R so that the desired track at the 90 deg mark on the RMI/DI. If left then turn Left.
4. You will be on track when the head of the needle moves to 90 deg.
NOTE: This method can be extended to use any intercept angle you like AND you can use it to get an approximate intercept heading to track from a given postion on a bearing to another bearing/distance eg tracking in to the station at a certain distance (say 10 or 20nm), cut the corner to intercept a track outbound at a given distance (say 20 or 10 miles or whatever).