Greetings @Anonymous I see that you are visiting as a new member to the Inventor Forum.
Welcome to the Autodesk Community!
Using Rip is much more difficult than Extrude so good effort, impressed you got it to work like you did!
Moving forward you'll want to use Rip for creating a long seam cut, primarily for a cylinder or rectangle for a flat pattern.
Here is a screencast showing a few different tips, cutting Distance using the Thickness parameter, cutting using the To and selecting a face.
I wouldn't use mirror unless you know the part will always be symmetrical. Alternatively you could create half the part and mirror the whole thing along the center to eliminate doing the Flange commands twice.
You can see I was checking where the XZ Plane was to use as the mirror plane, but instead had to create a Midplane between Two Planes.
On Sketch1 you would want to setup the middle of the rectangle at the Origin > Center Point for easy symmetrical use later on. Good habit to start.
If you get stuck again let us know, hope this helps!
Please select the Accept as Solution button if a post solves your issue or answers your question.