Hi, am learning about conversions in my general chemistry class and I have no clue how to cancel things like my

Hi, am learning about conversions in my general chemistry class and I have no clue how to cancel things like my

professor does while solving a problem. For example, we had a question where we had to convert kilograms to picograms and I simply do not understand how she knows where exactly to place the numbers in order to start cancelling and end up with the right answer. Would you please help me solve any conversion problem it can be from kg to picogram or km per hour to miles per second since that is what we have been learning in my chem class.

Thanks.