Ideally, you’ll already know how much the TO does, which limits how much you need to tell the judge about. If you can avoid overwhelming the judge with too much detail, that will make his life easier. In our scenario, the TO is scorekeeping, which helps a lot. Let’s say our TO is experienced with GPTs, and knows to have the checklists ready, land stations set up, and will handle registration and player entry in WER.

The technical aspects of judging Competitive REL:

Make sure the judge has read the IPG (with an emphasis on the Missed Trigger policy, since it’s different from Regular REL). Even if he’s not intimately familiar with it, just having a copy of it available will make it easier to handle rulings. Direct him to thedci.com/docs and suggest that he download some kind of judging app if possible from apps.magicjudges.org/forum/77/.

Make sure he knows the MTR. As an L1, he was required to know certain parts of it, but reviewing the sections about shortcuts and free/derived information would be especially helpful. Remind him about checklists, pool/deck registration and time limits, offering suggestions for the deck swap (paper bags, or rubber bands, etc.; collect and redistribute or some combination of passes not announced in advance).

Remind him that deck checks are required (at least 10% of decks in the event). Counting deck lists should be performed during the first round if possible, with any fixes/penalties applied at the start of round 2 (this can push to round 3 if necessary).

Provide the link to the GPT Fact Sheet www.wizards.com/Magic/Magazin e/Article.aspx?x=grandprix/trials and make sure he understands the tournament structure. Tell him to confirm with the TO whether the top 8 will be draft.

The other aspects of successful judging:

If you can, try to get a feel for how much the judge can handle before getting overwhelmed. Some will just want the most relevant information, and some will ask endless questions to prepare as much as possible. Follow his lead and give him what he needs.

Andrew suggested providing a checklist. This is great idea if you have the bandwidth to provide this service! (Perhaps you can provide a sample checklist which can be included on the Judge Wiki!)
As Paul pointed out, providing your cell number can be great if you think you’ll be accessible. The link to chat.magicjudges.org (#mtgrules and #mtgjudge channels) is an invaluable resource to get quick answers. Let him know that his rulings are final , and a wrong ruling isn’t the end of the world so much as a learning experience.

If you can, make suggestions for the announcements he’ll make at the beginning of the event (e.g., tournament structure, prizing, reminder that this is Comp REL and what that means, etc.)

Tell him to show up early for the event to make sure everything is in order. Be available for any questions leading up to the event. Reassure him that he’ll do fine, and there is a great support network for him. Remind him that he earned the chance to judge this event, and we trust him to do a great job!