Not sure on platys...
Here's the short on cycling....
When I cycled my tank, I did a fishless cycle using shrimp tied in a piece of pantyhose so it would be easy to remove at the end. I used a liquid test kit as they are more accurate; I have the API master kit which while a bit more expensive, is accurate. I let the shrimp rot in the tank to produce ammonia. I tested for ammonia, nitrites and nitrates. As the cycle sets up, the ammonia spikes and drop as the nitrites appear. This is followed by nitrites spiking, ammonia dropping to zero. Then the nitrates appear and spike while ammonia and nitrites drop to zero. The bacteria converts ammonia to nitrites and nitrates to nitrites. Nitrites are the least harmful to the fish. A cycled tank needs smalller, less frequent changes. When my ammonia and nitrites were at zero and my nitrates were at 10ppm steady for three days, I removed the shrimp, did a partial change and bought my fish.
Fishless cycling isn't hard, but it takes time so you have to be patient. You need a heater and filter on the tank. You need a source of ammonia and a test kit. Ask if you want to do it and have questions. :)
*Remembering Scout, Ninja and Rembrandt aka Remy, and Mac, our first bettas*
And getting to know Merlin
who lives in our 5 Gallon planted tank*