Day trading is very short-term trading, and it can involve holding an asset for just a second or ten, to a few hours. The main goal is to sell your digital asset before the end of the day, hopefully making a small but quick profit.
Building your own day trading strategy is a process of incorporating things from other proven strategies that you are comfortable with. A quick Google search will give you sites offering you cryptocurrency trading signals and crypto trading algorithms that will make earning your living as a trader look like a foregone conclusion. It is not. You should not trust people offering to do the work for you. Learn all you can about market volatility, about fundamental and technical analyses, how to read a candlestick chart, etc.