From beat ghillies
The gillies on each beat are normally willing to give some instruction, but this will depend on the policy of the beat, the helpfulness of the ghillie and what his other responsibilities are for the day you are fishing. Always check with the ghillie beforehand to see what the situation is.
Third party instruction on beat
There are a number of qualified instructors in the area, who you can pay to join you on your beat. However, be sure to check with your ghillie that he is prepared to allow this, and if he does, use someone who he would recommend. Never turn up with an instructor without having cleared it first, as this might cause bad feeling with the ghillie which won't do much to help your trip.
Local instructors and schools with their own water
Some instructors and schools can offer tuition on their own water, either informally on request, or as part of formal courses. These are normally run on waters at off peak times, to save on the cost of fishing, but there are some exceptions to this.
Pre-visit courses outside the area
Some people prefer to receive tuition before they visit the Dee, so that they can take full advantage of the fishing they have booked. Ask your local tackle shop for suggestions.