I think you need to read FAQ before you start to trade in Bitmex. After that, you need to watch a video about trading in Bitmex which is available on Youtube. There are so many good videos you can watch to learn how to place an order, and I think your problem is about setting the right number in the column.

This video is an example for you can learn:

Placing Orders, Setting Stop losses and Targets, Leverage and Margin
Bitmex Stop Loss Tutorial And Fees Explanation

I am sure there is many more good video for you to watch, you can get something from the video. I hope you can solve the problem, so you don't have a problem to trade in Bitmex.