Start by gathering reference pictures and comparing the current outfit to the Amazon one. If you are going to purchase a base to alter, make sure the base is actually going to save you time and hassle! If the costumes are significantly different from each other then it may be better to create the costume from scratch or build the costume from another garment.
If it is a good base, I suggest creating a list of what changes you need to make and what pieces are missing. This list will help you through the process! Once you have your list, start planning out each piece.
If you’re missing the mask, for example, how would you go about it? You could find a commissioner to make the mask, find a pre-made mask to purchase, buy a generic mask at the store and build off that or create it from scratch. Choose a method and then plan out what you need for it: If I want to commission it I will need to research commissioners. If I want to make it I may need to look up tutorials.
This is how my cosplay plans are usually structured:
You might not need to find tutorials, you may have an idea on how to accomplish something on your own. In that case, break down the steps or make a note in a way that works for you.
Once you’re done, it may look like a LOT of steps (depending on how far you’ve planned it out) but you’ve pretty much created the basic plan for your costume. You can cross things out as you go.
Personally, some of my lists get a little intimidating! So what I will do is create a master list (everything that needs to be done for the costume) and then choose a few things off that list as my goals for the day or week. For example: If you make the mask, you might find tutorials today and read through them. Tomorrow you’ll go buy the materials and start it. Breaking it down into smaller jobs/goals makes it less intimidating and makes the project seem a lot easier.
You can use this same method for planning out a full costume too. You’ll just have to break down the full thing instead of missing parts: cape, shirt, pants, gloves, shoes etc.
Hope this helps you get started!