I'm in the middle of doing something like this now. I basically broke it up into five different areas (four different areas that will be landscaped and the fifth area is a paver block patio that I'm going to extend so I can build a grilling area) and it should take me about a year to finish the project. The best advice I could give you is to get a couple of estimates from local landscaping companies. A family member of mine is a contractor and he sent over two guys he works with often. I was honest with them and told them that I would likely do it on my own and offered to pay for the estimate but they refused payment. They gave me their design ideas with plants, trees, rock, blocks and edging, and provided brief explanations why xyz plant would be the best option. I just took what I liked from each of their designs, did my own research and made my own plan.
I started by removing the existing landscaping in two areas and then laid my edging and retention wall down to section off the area. I have experience with this so it was easy for me but it's probably the hardest part if you lack experience. My suggestion is to watch instructional videos on youtube, use the proper base (helps you get a level finished product and keeps it level over time) and then take your time laying the block.
Started planting next. This is the toughest part for me. I'm not a green thumb and there are so many things to consider - color, size, space, sun exposure, shade, maintenance, price, seasonal changes, etc. I would have dicked this up if not for the designer's guidance. I used their color schemes and their suggestions of what to plant in sunny areas rather than shaded areas (used google too). Each flower is tagged with their fully grown dimensions so it is relatively easy to figure out if it'll fit in the space you have.
I ran lighting next. Placement of the lights is trickier than I thought and the designers did not give me any ideas so it was a trial and error. The path lighting is easy. The only issue is making sure you have enough light to see but not so much that it looks ridiculous. I installed what I thought I needed and then added a couple lights and changed the spacing. The well lights are trickier. You're typically going to install these during the day so it's impossible to know if the light is placed in an area that accents your plants. I had to adjust them several times before I got them right. It's not very difficult to adjust them. Just make sure you do not cover the area with stone or mulch until you get it right. Spot lights and flood lights are just as tricky. Install them and then walk around the house/drive past the house so you can see it from all angles. You'll adjust it over and over, I can assure you.
Stone or mulch is last and it's easy. Mulch stinks, literally and figuratively, but it's cheaper.
Drinking beer outside is my hobby. This landscaping shit just gives me an excuse to do it.
