A strategy is to apply two moving averages to a chart, one longer and one shorter. When the shorter MA 50 day scrosses above the longer term MA 200 days it's a buy signal as it indicates the trend is shifting up.This is known as a "golden cross."
When the shorter MA crosses below the longer term MA it's a sell signal as it indicates the trend is shifting down. This is known as a "dead/death cross"
For cryptocurrencies use 4 hour charts.