I've had cleaners (mostly v successfully) for 25 years and I would say
Ask clients how big their property is and set the cleaning time based on the number of rooms (it might be fine to do 2,hours a fortnight in a 3 bed house if you're just doing a couple of rooms), some rooms can be cleaned on rotation, some need to be done every time.
Have a guide for cleaners and clients of what a standard clean of each room involves. If they want extra stuff done, like oven cleans, bed changes, or a mad amount of ornament dusting, that takes extra time.
Tell them that surfaces need to be clear, bed linen and ironing (if you offer that) needs to be laid out etc - if the cleaner also needs to tidy that takes up time. If that's what they want, fine, but estimate the extra time then review with the cleaner.
Tell the client they need to have X products and tools. Cleaners to carry a small stash of emergency supplies so a clean doesn't have to be abandoned, but make sure the cleaner reports immediately when they need replacement products bought or the hoover isn't working.
Have a clear system so cleaners and clients can quickly report problems to you. (maybe there's a standard app you can buy into - this also helps deal with cryptic notes from clients and cleaners who may not have English as a first language.)
Ask clients if they need a specific window, or the cleaner can just pitch up any time in the morning or between 9 and 5 etc, then organise the cleaners day so they have some flex on times.
Use a clear security check on cleaners and tell the clients what it is. If the cleaners are holding keys, what is your system for that. Presumably an air BnB style lock box might be an alternative you can offer some clients.
You need to work out insurance for breakages and tell clients what the system is (to avoid the loony poster above who thinks her cleaner should replace a £100 candle)
Tell clients how much of what they pay goes to the cleaner, and relate that to london living wage and that what they pay also covers insurance. Everyone is always a bit surprised about how much cleaning costs, so this helps.
Have a welcome back with all of this info in it
If you want to offer printing, make sure the cleaner can iron. Come to that make sure they can clean.