You do not buy/sell at one price. You make a prediction and if it goes in your favour, you increase your position. If it goes against you, you close it immediately. Your mistakes will be less costly and you never even try to catch the bottom/top
If you are in a bear trend for several months and have a neutral pattern, your standard assumption is to short. People might think it is too low already to short, but until proven otherwise, nothing is too cheap to sell
Let's say you have 1000$ and do not want to lose more than 2% (20$) on a trade. The asset you trade is at 10$. If you take 100$ and open a long trade with 5x leverage (500$), you have to set your stop loss at 9.60$ (-4%). 500/10*(10-9.60)=20
Let's say in the example above we have significant support at 9.81$ and we set our stop loss to 9.80$. We could therefore use 10x leverage. Our position size is 1000$. We can store 90% off the exchange while having a position as big as our bankroll.
If 10-20% of your bankroll would substantially change your life or fullfill a dream of yours, take it off the table. Greed will probably bring you down at least once. It is good to look back at what you have achieved and earned in the past.