Start with the feeding rule, in writing
Overfeeding is the one mistake a kind-hearted sitter is most likely to make, because fish beg convincingly and a pinch looks so small. Your plan's first line should remove all judgement from the job: the exact food, the exact portion, and the exact time, for each day of the weekend. Measure the portions out in advance into small labelled pots, one per day, so your friend never has to guess what a portion looks like. Add the sentence every sitter needs to read: if the fish still look hungry, they are acting, and extra food is the one thing that can genuinely harm the tank before you get home. If a day will be missed entirely, say so plainly - most healthy adult fish cope with a skipped day far better than with a double portion.
What runs itself, and what to simply check
Most of a settled tank looks after itself for a weekend, and the plan should say so, because a sitter who thinks they must operate everything will fiddle. List what runs on its own: the lights on their timer, the filter, the heater, any air pump. Your friend's job for each is a ten-second check, not an adjustment: lights came on, filter is humming and water is moving, heater light behaves as usual, water level looks normal. Show them where the plugs and switches are before you go, and mark clearly anything that must never be unplugged. A sitter who knows the difference between checking and operating is a sitter who leaves a healthy tank alone.
The touch-nothing list
Write down, explicitly, what not to do; sitters improvise most when the plan is silent. No water changes this weekend. No cleaning the filter. No rearranging plants or decor, no topping up with water from the kettle, no new fish as a surprise, and no food other than the pots you left - not bread, not a different tub from the cupboard, not a treat from their own tank at home. If a plant comes loose or a snail climbs out, both can simply be left for you. Framing the list kindly helps: the tank is balanced as it stands, and doing less is doing it right.
One clear emergency line
A plan fails a sitter if every small oddity feels like a crisis, so give one or two bright-line thresholds and a calm action for each. Fish crowding the surface gasping, or a filter that has stopped: check the plug is in and the switch is on, then phone you. Cloudy water with fish hanging listlessly: phone you before doing anything else. A leak: towels down, electrics off at the wall if water is near them, then phone. Leave your number where it cannot be missed, plus your vet's number for the rare case of an injured fish. Handing the whole plan over through a KinsLe Care Pass gives your friend temporary access to the tank's routine, feeding portions and key details for the weekend itself, so the plan lives with the tank rather than on a scrap of paper.