Hello again,
So start off making the combo box, add the different stages to the dropdown values.. Now, on your submit button, add a rule that sets the combo box value.... ie.. Set field value...
Set field value = 'Manager Approval'
You don't necessarily need the combo box, I just find it a good way to keep track of what stage values I use within the form... Now you can add a new section with your approval information to the form, and make it so that its only visible when Stage='Manager Approval'... on the manager approval button, set the stage field again to another stage, etc..
This is the approach i've been using, been working well for me.. but I think you can also use Sharepoint workflow..
Hope this helps.
HG