A lot of what I learned was by trial and error. And I knew that learning that way it would take more time to do the jobs, because we were learning and we'd have to redo things or it might go slowly since we weren't as familiar. So at first, I only took on very small projects, so that if it went sideways, I couldn't loose too much money spending more time to make it right. So first step is only take on very small jobs and use those are your training.
Below is a photo of the very first paver patio we did in-house. It isn't much. Just a simple rectangle but it was okay for our first attempt and we learned a lot. After that turned out fairly well, we felt more confident doing a little larger patio. The next one we took on was a little larger. Then another one a little larger. Then we started doing a little larger patios with curves in them. Then after a while started learning how to do steps with pavers. You just go slowly and learn a little as you go. Don't take on projects that are too far outside your comfort zone.
One thing that helped is that for jobs that I felt were way too large for us or outside our comfort zone, I would sub-contract those out to another contractor who was good at those. Then we'd be doing the landscaping around the patio or wall he was building but we'd be watching him and his crew the whole time - picking up tips and tricks of the trade the entire time. After a while, my guys came to me and said, "Jim. I think we could do this. You don't need to use that subcontractor anymore. We can figure this stuff out." So I still kept on using the sub for larger projects but for smaller projects I'd give those to my crews and the better they did, the bigger projects I'd give them.
You can learn a lot from your supplier too. Maybe you've figure out how to lay a paver patio but you can't figure out how to build the steps going up to it and make it look right. Go to your paver supplier and they'll be glad to help you figure out that. They can show you schematics, photos, examples, sometimes even come out and help you get it started. Choose a local hardscape supplier, find out who the outside sales rep. is, have lunch with him, get to know him, and then use his knowledge as you get jobs. He can be a huge resource for you.
Then the final place you can learn a lot is online forums. We have a hardscape forum here on lawnsite. You can learn a lot by posting your photos as you do jobs but you have to be aware that there are plenty of jerks in there. Sort of like the guy in this thread who keeps criticizing you because of the mulch you use in your area. Nothing wrong with using that stuff if that's what's common in your area. But there are plenty of Jackasses who think that because a certain practice isn't done in their area, that it shouldn't be done anywhere. You're going to get a LOT of rude guys spouting off their strong opinions in the hardscape forum here. There are other hardscape forums around the internet that are much more polite too. But online forums are great places to learn, if you can take all of the crap they throw at you.
Finally, just look around at your competitor's websites, websites of guys here on lawnsite, do google searches, etc. Get familiar with how other hardscape contractors have done jobs and you can pick up a lot of ideas that way as well.
It's actually a lot less difficult than you think. I remember just 6 years ago we were doing our first paver patio and we didn't do a lot of other hardscape work at that time either. Today, it's probably the single biggest thing we do. We are doing at least one big hardscape job every week of the year. During the busy season, we're doing 3 or 4 sometimes. Once we started to figure it out, it wasn't that hard. And our workers love hardscape work compared to landscape and irrigation installs too. So it's been a good thing that we pressed on and learned it. You can do that too. Just start small and work your way up. Good luck!