Working at Rexall - Cashier Rexall Employee Review

I cannot understand exactly what they want from me when they ask me to complete a task. I finish the task, but then end up saying, "you should've done it like this" when they didn't even state how to do it like that in the first place. They aren't thorough with telling me exactly what it is they want me to do, only giving me the overall idea but not the entire task. They also don't tell me the exact or common rules or procedures that I should know when working the cashier.