Build. Then build some more. Then, when you think you're done. Build more. As in anything, you get better, the more you do it.
Look at, watch and listen to builders you enjoy and respect. I learn something new, every single day, from other builders.
When looking at blueprints, try to follow the rule that in game tiles are 3' x 3'. (Making them 4.3' on the diagonal) That will help you follow the basic idea of the floor plans you are looking at, easier to do and make your rooms more realistic sized, for your builds.
Wall heights, I usually consider to be; Short - 8'-9', Medium 10'-12' & Tall - 14-16'. I've not found any info on what they are actually supposed to be, but that feels right.
When working a roof with several sections, you usually want to try to adjust the roof heights, so that all of the roof eaves match up in height and don't look all scattered and jagged. You can hold alt and raise or lower your roof, for fine tuning.
Be careful with MOO. I'll often save my build to my Library, turn MOO off and then plop that home on another lot and check what's gone wonky. If there is an issue, I'll demolish the copy, go back to the original with MOO off and fix the problem, then repeat the process, to make sure all is well.
Before you save to the Gallery, run a, or some, Sims through the house and make them touch and do stuff, to make sure everything is accessible.
Some people like hoarder levels of clutter. Some people like minimalism. Some people are more centrist. It's all your call. What make you happy.
Look at interior decorating websites. Search for the style of home you are building (and time period) and get some ideas from people who actually make money doing that sort of thing. You'll be amazed at the inspiration you can pick up.
Never be afraid to ask questions. You are surrounded by the most incredible, helpful, loving and kind Gaming Community, on the Planet. Use that.
Have fun! Simply enjoy what you do. Yes, sometimes there will be builds that cause you to lose sleep, sweat and use bad words, but in the end, they are yours and if you can upload them with pride and bring joy to others, it was all worth it. ♥♥♥
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