As someone who went from 2 stars straight to 4 stars after trying for 3+ days, trust me when I say don't listen to Isabelle's suggestions, they are just suggestions, and not helpful at all
Here's what you need to do or have
* 1x bridge
* 1x slope
* 8x villagers
* a few fences and DIY furniture
* a few flowers here and there.
Since I went from 2 to 4 stars, and the only difference was +1 slopes and +1 villagers, it may be argued that you can have 7x villagers as long as you have 1x slope, or the other way around, 0x slopes with 8x villagers.
The first few days I tried going from 2 to 3 stars while having 7x villagers and 0 slopes. I had placed about 300+ fences, and over 70 DIY furniture outside. Depleted nearly 100% of all my resources doing so, and for nothing. I also had placed over 200 flowers, though I unlocked a lot of achievements for the planting and watering of that amount.