Start by slicing your ciabatta loaf horizontally so you have a top and bottom half. Set both halves cut-side up on your cutting board. This gives you a clean, flat surface to build your layers on and makes it much easier to fold everything together neatly at the end.
Take your thin prosciutto slices and drape them gently across the bottom half of the bread. I like to fold the slices slightly as I go rather than laying them flat, which gives the sandwich a bit more texture and volume once you take a bite.
Lay your brie slices directly on top of the prosciutto. The warmth from your hands as you assemble is usually enough to soften it slightly, but if you want it extra creamy, let the cheese sit at room temperature for about ten minutes before slicing.
On the top half of the bread, drizzle the balsamic glaze followed by the olive oil. Then pile on the arugula. This step is where all the flavors start coming together, so don't be shy with the balsamic if you love that sweet-tangy kick.
Close the sandwich by pressing the two halves together gently. Use your bread knife to cut it in half or into quarters, depending on how you plan to serve it. If you're making this for a group, slicing into smaller pieces makes it easier to share.