just like you would any shelf... one shelf at a time. measured to fit the angle, spaced to your needs.

I would look into roll out drawers for the bottom and and simple shelves above on side cleats. Baskets on the shelf above the roll out might help to access your storage.

How deep and how wide and how tall is the interior?

I'd consider baskets attached to the inside of the door. (Elfa back of door storage.) It will be easy to stand right in front of the basket when the door is open and see what you are reaching for in a hanging bin instead of diving to the back of a shelf.

We have these in many different sizes for what we use as an "inside the door" medicine cabinet/linen closet. Some are small enough for bottles of pills, vitamins, creams to line up on without getting lost. Others hold extra shampoo, travel cosmetic bags, hair dryer.

Figure out the depth these would take up when the doors are close. Then you can organize in the open space that is left in the closet with a way to stack towel and store toilet paper and other big items.

IMO we need to know the size of the space and also pics from further back showing the whole space . it looks like the door hits the toilet so IMO no door for sure and some nice shelving left open so towels stacked, pretty jars filled with bath salts and scrubbies and some solid baskets for other stuff needing storage . IMO usually to much stuff stored in bathrooms that really do not need to be there . I need to see the whole space please Post the pics here in comment s.

Wow! What an awkward space! I would just use it for stacking towels. Cant you put all of that other stuff under the sink?

Other than than , put up some shelves.

I have a bathroom closet like that. I have some of those freestanding shelves, like these:

VARIERA shelf insert, white, 125/8x51/8x61/4" - IKEA

I also have things in bins with lids, so they can be stacked. I label them. And I have some back of door storage. It's not elegant or worthy of a picture, because it's my daughter's bathroom. But it uses the space better than you're using yours. I do have a separate, equally awkward space for toilet paper and cleaning supplies, but no undersink storage.

Bins make it much easier to grab stuff in the back, because you just pull out of the front bin and then the back bin, rather than moving many bottles of shampoo, hair spray, and other stuff. You can organize bins by family member or type of item.