There are only a couple of achievements in House Flipper 2 that aren't very straightforward. One of these is finding Reva's journal in the level called More light, please.
Location of Reva's lost journal in House Flipper 2
When you begin the level called More light, please in Coralroot Forest, Reva will call on the phone to talk about what she wants done to her little summer home in the woods. During the phone call, she mentions that she's lost her gratitude journal and is hoping she just left it at the cabin. Picking up the journal while playing this level will earn you an easy achievement, so it's good to pick it up if you happen by it.
Weirdly, there are two locations where you can find the journal; it seems perhaps that this might be some kind of bug, so I will list both locations here in case one of them is removed in the future. Both of the journal locations are in the master bedroom upstairs. The first is in the back corner of the room on the window-side of the bed. The second is located on the floor in front of the dresser. In order to pick it up, you just need to line it up with the reticle with the journal and press the E key. Once you pick it up, it will complete the achievement.
For more House Flipper 2, check out House Flipper 2 – All platforms and release dates on Pro Game Guides.