Here is where knowing your cat is good. Most cats enjoy being high up. A few are glued to being down low. Most cats enjoy some kind of "container" (is he crazy about sitting in boxes?). Some cats love something to hide inside. Some prefer to sprawl and would rather have a large platform. Most like to scratch. Some are very agile and need minimal arrangement to go up the shelves, while some are more likely to need something easier to go up.
It all depends on the cat. I made the first tree when mine were young, so I just included lots of things. Some things they nearly fought over (the high enclosed warm "nest") and some they still don't care much about (the platforms inside the tall barrel that make a series of rooms they can climb through - my cat uses it only when she doesn't want me to be able to reach her, lol, which is once every couple of years).
If you want to build it yourself, I'd start trying to figure these things about your own cat, and start googling images of "catification" or homemade/DIY cat furniture/trees/towers/shelves.
Shelves are great. They usually need to be staggered, or if they are long enough they could have a hole cut in each one, allowing the cat to go through it to the shelf above. Or you could provide a carpeted ramp (or ramp sections) but cats will often just leap over them.
If he is 10 years old, you might want to design it with an eye to being sure he has a way to climb up if he needs that in future. It would be a shame for him to love his new spot, and not be able to get there anymore.
If you have just one cat, many multiple perching locations are probably not necessary. If they are identical, he is likely to choose only one. It might be the highest, or the one at the height he is most comfortable at, or that one that gives the best view of something he's interested in, or the one he can reach out and touch you from.
But if you give him ten the same, he may use only one, or two.
But different things appeal to different cats, or sometimes at different times. You might include a closed nest, an open platform, and a hammock, and he may use them all (depends on the cat).
Something to scratch, usually low, and always stable, is a good idea.