First, open the Measure app. Once you open the app, make sure the black bar at the bottom has selected "measure." If you aren't sure, don't worry. It selects "measure" by default, so just make sure you don't click "level" on accident.
Here is where the app will be scanning for furniture since it's super handy for measuring bookshelves and other furniture. Move the iPhone around just a little, so that it realizes there is no furniture in view (if there is that is okay, just do your best to measure the wall with that furniture there).
Here is where you want to be careful about accuracy. To make the first point, make sure you have a good angle and get as close to the corner of the wall as possible. You also don't want to be too far away or too close to the corner.
Once you have found a good angle and distance, point the camera at the first corner. I like to move from left to right. Click the plus sign "+" to add the first point.
Now, slowly follow the wall where it meets the floor and when you get to the final corner, hit the plus sign again. You will have a line that looks like pictured.
In my case, I want to see what size bench I can fit in front of this wall. So, I am just doing its portion of the wall.
However, if your wall has posts like mine does, and you are trying to measure the entire wall including that section, just make a point for each corner. If you are measuring the entire wall or room, you just continue turning and making points in each corner until you've captured the entire wall or room.
Once you have gotten all the points, you will see the app measures by feet and inches. This section of my wall, for example, is three feet and two inches.
If you feel that something is off in the measurement, it may be the angle and distance from the wall while measuring that was the culprit. If so, give it another try. You can also measure straight across the middle of a wall, but I find this method more accurate.
Voila! That's it. Of course, the bigger the wall, the bigger the room, the bigger the job of measuring it. You can even measure the wall from foot to floor if you are working on a project or gallery wall.