First of all - well done for considering DSC2, and best wishes as and when you embark on the process.
As to your question, during the gralloch you will be expected to check the mesenteric lymph nodes when inspecting the gralloch.
As part of your Individual Cull Record (ICR) you will also be required to remove the head and hooves of the deer, so you should also be comfortable locating and inspecting the sub-maxillary and retropharyngeal lymph nodes.
The Witness needs to see the above.
You are also expected to know where and how to locate and inspect the portal, gastric, bronchial and mediastinal lymph nodes for any abnormalities.
If you can work your way from head to tail on the deer finding, examining and describing the above lymph nodes, you should be fine.
IMHO the best way to do this is to become familiar with locating the lymph nodes beforehand from a practical - rather than a theoretical - perspective. The witnessed stalk is not the time to be digging around trying to identify lymph nodes for the first time, not least because as a Candidate you have to be able to find the lymph nodes without prompting or guidance.
My other three tips would be as follows:
First, there is a lot of information on DMQ's website. Download it and spend some time reviewing it well before you go on your witnessed stalk, as it will help you to understand fully both what you need to do as well as what the Witness will be expecting to see.
Second, become as familar as possible with all the Performance Criteria (PC's) and their respective key features. As stalkers there are lots of things we tend to do automatically, born out of familiarity. However on a witnessed stalk you are being witnessed as to whether you meet the pre-defined criteria or not - so be confident you know the criteria and can show them as having been met.
My final tip would be to spend enough time preparing for Element 4 "Transport and Store Dead Deer". For obvious reasons the primary focus on an outing tends to be on successfully finding, identifying, stalking, shooting and gralloching the deer. Then we all tend to breathe a huge sigh of relief. However......there are a large number of PC's in the ICR that follow on from that, including transporting the deer back to the larder, lardering the deer, the use of the larder, and maintaining accurate cull and larder records. These criteria may not be as glamorous as the stalk itself, but they are criteria that have to be met just the same. Become as familiar with these as you are with the task of grassing the deer.
Best of luck.