2 solidworks will pick the rip edges for you by process of elimination.
Convert cylinder to sheet metal solidworks.
If you can insert an axis the model is an exact cylinder.
3 when all edges have been added to the two groups hit the green check.
Here is your sheet metal box.
This often means making your first feature a base flange but it can also make use of the convert to sheet metal command if your part is imported into solidworks or if the shape is easier to model as solid no need to use the shell command but more about that in another tech tip.
Sheet metal convert and flat pattern suppressed.
As a test try to insert an axis on the cylindrical face.
Notice the three features added.