Create a Modal from a current page

I currently have a web page that I need to make into a modal. I have a button on a first page that, when clicked, should open up the modal and allow the end user to select a 'Standard' from a dropdown. Once a Standard is selected, the end user will see a hierarchical tree and should be able to drag and drop leaves to a right-hand column. Table updates need to be performed with leaves that are dragged to left or right hand columns. This modal needs to be styled in accordance with our style guide. Current page works with only 1 Standard so I also need to know why it does not work with other Standards. An image of the current page is attached, although it needs to be styled and made into a serviceable modal. Should be able to work afternoon US hours.