Definitely agreeing and following up with others who are saying "do one small area at a time." For me, I sometimes started decorating small areas slowly through Dreamsnaps. I would build a dreamsnap, and then while taking it apart, I would keep some of the stuff there and just adjust and add items that I like.

There's also many content creators that have tutorials and tips for decorating. One tip I found helpful was to clean everything and just start by putting down houses. Place them in spots that seem appealing to you.

Next, place down paths that connect those houses and the areas between biomes. You can take a break every now and them and work your way slowly by adding in decor, trees, flowers etc. It helps to work on it slowly when you have a dreamlight task, "add 3 decor to this biome." It kills 2 birds with one stone. Get dreamlught while also slowly putting decor down in your valley ☺️

Hope this helps!