1. Master the brick selector menu. Being competent with this GUI is an overlooked but extremely necessary skill. Get a comfortable system of favorites that you can flip between when you need bricks. Scrolling through the brick menu for a single brick you need is tedious. Get TMBI or infinite scrolling if you want.
2. Spend an unreasonably massive amount of time on everything you build. Look at every wall, floor, ceiling, doorway, piece of furniture and think, "How can I improve this?". Edit it, and then ask yourself, "How can I improve it even more?". Keep doing this until you can't think of any way to improve it. Do this for every single detail in your build. At first, this will be laborious and discouraging, but eventually you'll build up a visual library in your brain that can be quickly referenced, and these good styles will just incorporate themselves into your personal theme.
The goal isn't actually to be a great builder. Everyone thinks that being able to build something that doesn't look like total garbage is a magical skill that takes veterans 10+ years to master. It's not. If you spend 30 minutes building a box today, you will have made an incredibly above-average box. The actual goal is to learn how to build fast and maintain a good style, so that you don't get discouraged with your projects taking too long to complete.
3. That being said, get comfortable with the new duplicator and all it's features. There's a lot of clever functionality in that add-on that will cut the time required to do basic tasks down to "instantaneously".
