Your steps aren’t quite right. You’d actually end up setting the variable to the value of itself…
But first - to answer your question about when to create the global variable - global variables exist within the project itself. So you “create” them within your RP file: Project - Global Variables…
By default Axure creates a “OnLoadVariable” variable. You can create your own, too and name them whatever you like. You can also set a default value for the variable. I was being somewhat lazy in my example for you and just used the default variable.
As to setting the value… In this case specifically, you only need to use the “OnSelectionChange” event for the dropdown. Set it so that it sets the value of your variable to the selected value of the checkbox by assigning a local variable (LVAR) to the selected option of the dropdown. Then set the global variable to that value. To see the logic in my example file, either open the “Dropdown” master from the masters panel, or double click on the dropdown on any of the pages. You’ll hopefully see what I’m talking about. Everytime this dropdown value is changed, the global variable will be updated with whatever the selected option of the dropdown is.
Regarding linking between pages…
In my example, the “linking” is done by putting an OnPageLoad event on a master. If you aren’t familiar with masters, it’s basically just a way to reuse the same functionality in multiple occurences. If you look through my sample, you’ll see every page has an instance of the master. The “OnPageLoad” event for the master will fire every time the master is loaded and will set the selected option of the drop down to the value of the global variable.
Hope this makes sense - would be much easier to walk you through this in person!