Ok, I might be wrong here, but I did my best. The cube itself is fairly straightforward, but that gap is really tough to get right. I tried to illustrate the guidelines I'm using to show where my thought process is.
One thing that is very important and I feel is missing in the exercices, is that the shadow goes along the perspective lines. Those perspective lines will help line up where the shadow should fall.
On regards to marketing for comissions, that is really tough since I don't really promote myself. Eventually I might, but it's not my focus for now. I can think of what I do in my non-art business that might help:
1) Selling is an art. Having a good product is barely the beginning. You do need to have a pipeline to allow clients to reach out, for you to deliver, and for them to pay. You want to find an audience willing to pay and use techniques to sell to them, and make them feel good for buying.
2) You have to be willing to do what people are willing to pay for, not necessarily what you want to do. Figuring out what people are willing to pay for is usually the biggest challenge.
3) Donāt undervalue yourself, check standard prices, what you should charge yourself based on your effort invested, and set a base price. It is useful, however, to offer initial discounts to get the ball rolling. Itās always easier to āremove a discountā than to āincrease the priceā.
4) Just like in art, I think there are 2 good ways to go at it:
a. Copy what people already successful are doing.
b. Be persistent. Success is built over many failures.
5) Marketing. Getting your name out there so people see what you offer and check your prices. There are many videos out there with good tips, and what works is different for different people. Some post fanart in reddit or discord. Some go to conventions to sell physicals, entering contests to get their name out there. I'd recommend looking at some videos about it, take notes, and trying them all and see what works for you.
There are a few examples that come to mind worth checking out. I think these folks have a nice setup.
https://www.zlide.de/
https://anton-portfolio.carrd.co/
https://artistree.io/itsanton
https://patrycjalerch.carrd.co/#