I would suggest folding your background into 'workable' areas (1/4ths maybe?) and lightly pressing each fold to get your 'centers' of each region. Then figure your placement for the scrolly designs within each area, fold in half (or other dimensions) and pin or pinch fold them to get your 'mark'
I have done similar applique and that was easiest for me.
Remember for every one way to do something - there are others!
Do show us your finished medallion please! Love this look!
What are you going to put around it and how big are you going to finish it off at?