Trying to have a value in my winform textbox written to one or more selected cells in excel.

Scenario:

1. Open an excel file using windows explorer (so no coding here)
2. Select a cell in this excel file's sheet( I know selecting multiple cells could be a problem, but I want to solve that later on)
3. Open my winform app (let's call it LookApp for reference)
4. Do stuff in the app so that a textbox contains a string(let's call the string ProductCode for referencing)
5. Press a button on the LookApp
6. The string Productcode is inserted into the selected cell
7. Excel gets focus over LookApp

Obviously, I can handle step 1 to 5. But I've searched the web far and wide and can't find a snippet in the direction of coding step 6.
Probably easy to code step 7 once I know step 6...

I am new to coding with MSOffice, or outside of winform/database anyways.
I have refenced MS Interop in my project but don;t know how to use it :(

I really hope you can help!