Hi growstudios and welcome to the forums. This is a topic that has many approaches so it depends on which you prefer. The approach I like to use is this:
If you are only targeting 16:9 ratio, it is simple, just select the screen resolution that you prefer and work from there.
If you are setting up for multiple aspect ratios, I usually select 2 or 3 of the most used ratios, create a bounding box with those ratios, and set them in the middle of the layout. All boxes are transparent in the middle, they just have edges so I can see them. From there, I design my game so that
a) All the vital parts can be seen from all the different ratio boxes. (the intersection of all ratio box)
b) No unwanted objects can be viewed from any of the ratio boxes. (every part of all the ratio box)
For example, a vertically longer aspect ration will show more of the floor than a shorter one. So it is important to make sure that the graphics don't cut off suddenly or display things that are not supposed to be seen.