First off you need to get a container for it, anything large and relatively shallow (few inches deep) will do, i chose this ornament as it was there

this is it after i recently cleaned it all out as there was a 1cm thick layer of sediment on the bottom as its been running over a year and had had the water for longer

old tank water is perfect

I used pond though as it was just behind me lol
You now need a constant food source in the form of suspended algae (green water) this is mine, it gets old tank water, mainly the bits that i've squeezed the filter pads out in as its nice a polluted with organic particles, high levels of ammonia, nitrite, nitrate and phosphate, perfect for algae. Dead plant leaves from the tanks also go in

nice and green lol
Next you get your daphnia to start your culture, any lfs live stuff should do, but may not be enough unless they are decent portions so maybe find some in a local pond or something, but i didn't say that
This is mine, started with store bought daphnia

you can just about make them out in the video below, last year i even had a mutant variety start reproduceing i think as they were flourescent green

you only need to chuck some green water in every day/other day
In the summer it is mainly asexual reproduction (cloning) by the females as it's quicker so more can be pumped out to quickly populate the area

but in the winter dont try farming them as it's all sexual as they want to get hardier young that are used to the cold so they need to mix the gene pool somewhat, this is slower so they wont populate as fast

but it does mean that they'll still be there when you go back to them in the spring
Any questions then please ask