Hi! Jumping in the discussion.
I'd just like to add, you can use the Q key to make it easier to make a square or rectangle when mapping.
Regards and happy mapping!

Hi! I'm jumping in the discussion since I used to have a hard time identifying which ones to map and how to draw a perfect rectangle for the buildings.
The links that Frans posted are really helpful, especially when starting out in OSM.
Just to add, when I am not sure with the shape of the building I am mapping or if I want to verify how big the building is supposed to be, I usually switch between imageries just to check. I also tend to look for the black shadows of buildings to see if I am accurately mapping.
Hope this could help you out and hope to see more edits from you! Regards and happy mapping!

Hi! Thank you for this feedback! I did this when I was still starting in OSM and was confused how to really map the buildings. I will surely take note of this.
Best regards