Day trading is a very short-term trading. An asset may be held for just a second or ten, to a few hours. The main purpose when using this strategy is to sell your digital asset before the end of the day, hopefully making a small but quick profit.
To form your own day strategy you're supposed to gather techniques from other proven strategies that are suitable for you. Be careful with the people in the Internet who may offer trading signals or other algorithms that as they say would make your trading profitable. To make it actually profitable you should learn all you can about market volatility, about fundamental and technical analyses, how to read a candlestick chart, etc.