You could try one of the myriad of online landscape/garden design courses offered. I don't have any personal experience with any so cannot recommend one and unfortunately also don't have any tips on how you can evaluate their quality. But there must be at least a few that would be a reasonable alternative to attending a brick and mortar school. And certainly a convenient alternative for someone with a 9-5 job and a busy family life!

This link may help.

Gardengal's suggestion of an online design course (I have no experience with them either) sounds like a good possibility for learning the landscape design PROCESS. The process begins with site inventory and analysis, and moves through schematic and conceptual phases to various forms of construction and presentation drawings. One must also learn about managing the project. These things are fairly straightforward, without much variation in how to accomplish them. When it comes to the artistic aspect of landscape design, I think this is much more difficult to teach and actually, the weak point in instruction. The rules for "good design" remain vague, or at least, to me, appear to. What I'm saying is that, in fact, you may end up teaching yourself what looks good and why. Therefore, it's not too early to begin to observe and study as much as possible. Notice why some things work well and why some things don't. Notice proportions ... connections ... materials, etc.

I have no idea where you reside or what kind of work you do. But you might consider obtaining the highest level position you can in a local landscape company and begin to absorb all the knowledge you can about the work. Knowing HOW things are done can help immensely insofar as design is concerned. Also, it may give you opportunities (that you can't see now) that lead to a beginning level design position, where you can work and learn under the tutelage of a more experienced designer. It may begin with just "helping," but depending on your abilities, your help may be highly welcomed. At least you'll be getting some pay, as opposed to the poor students who must pay others! Another things that will be necessary to be a designer today, is computer graphic skills. Somehow, you'll need to learn CAD and other drawing skills. You could begin with learning Sketchup as it is used much.

I'm guessing the OP is in the UK.

If the OP is located in the UK, there are a number of well respected online or distance learning design schools. I recall some detailed discussions about this topic back in the pre-Houzz days when this was much more of an industry discussion forum rather than a "help me design my garden/give me curb appeal" request board for free design services. Unfortunately, I don't remember any specific names. But you can just look through an issue of Gardening Illustrated to come up with a dozen or so.

Hi James,

I agree with everything mentioned. However, I want you to get one customer. One client. Start with your yard and landscape. Make it beautiful.

This is going to take time. Years in fact. If you need income right away, go work for somebody part time.

Build your portfolio. It starts with one client. Your portfolio is your business card. Your reputation. Tell everyone you know you're a landscape designer, looking to help people love their outdoor space. Offer friends and family free consulting. Take pictures, become a local authority.

Do you love plants and being outside? It's a must. Do you have good taste? I bet you do. Can you read a few landscape design books? Yep.

Good luck! Don't over think it. Get out there with your shovel!