Like most tasks, taking the time to plan out your goals and steps is extremely helpful when cleaning and working with data. Knowing exactly how you will achieve your goal before you start allows you to be much more prepared and prevents you from running into a wall and having to try multiple solutions for your problem.
Goals
When data prepping the first thing to do is know exactly what you need to create, if you are doing a data task such as an Alteryx challenge you can compare your input to the desired output to find out what needs to be done, for example if you need to replace nulls, create new fields or change any of the metadata.
Execution
Once you know what needs to be done you can go about deciding how you will achieve it. Decide which tools or steps you will need and how you are going to use them so that when it comes to creating your workflow you are not still trying to work out which tools are necessary.
Order
Finally create an order of when you will perform each task so that you have a step by step guide for your data prep. Once again this will allow you to avoid going back and rearranging your workflow once you have started it.
Having a plan like this will ensure you are well prepared when starting your data prep and most likely ensure you can achieve the goal you are trying to get to.