Don't fixate so much on the other plane that you lose overall perception of your surroundings (terrain, obstacles, landmarks)
If you're flying wing, ALL you should be focusing on is the lead aircraft. Whatever lead does, you mirror. It's almost as if you're one unit. Taking your eyes off of lead can result in losing visual contact or possibly a mid-air collision.
How closely is really a question I was hoping y'all could answer lol. It will be at night and I'm a pretty new pilot. He's fairly experienced, so I'm sure he'll have some ideas when we brief tomorrow. When following him, it will be just me on board. And no on board traffic advisories.
I was planning on just leaving plenty of distance, but that video is making me want to follow close enough to get a good view.
If you've never flown formation before, I would not recommend your first formation flight be at night, alone. Find someone with experience, such as Cpt_Kirk, and let them show you the ropes and act as a safety pilot.
Have done it many times, if you are going to do it much more than an hour it can be exhausting for the one responsible for maintaining visual.
It's exhausting during the daytime, it's probably more significant at night.
Some other things to consider:
What types of planes are being flown? You need to ensure that cruise speeds are briefed ahead of time. You wouldn't want a 172 flying at normal cruise as lead with a Mooney trying to fly at normal cruise as wing.
As others have suggested, have a frequency you can talk on, and a backup frequency. If you're within a mile or so of each other, you should get flight following and identify as a flight of two and wing should not squawk on the transponder. At night, depth perception is already reduced so attempting to fly in formation could lead to other issues.
Formation flying is a lot of fun but it must be done safely. If you've never done it, do it a few times during the day to get a feel for it. I recommend either not flying in formation or taking Cpt_Kirk up on his offer.