You'll want to start by planning out your arrangement. I like to place them in their approximate locations while still in their pots so I can move them without making too much of a mess.
Pull the succulents out of their pot and gently remove as much of the soil as possible. If you happen to pull off some of the roots in the process don't worry! Your succulent will survive just fine.
This will ensure you have enough soil to fill the planter without adding more later, but also allows some wiggle room to put the succulents in without the soil spilling over the edge.
I've found it's easiest to start an arrangement by planting in the center. In this case, I planted the larger Echeveria in the center and nestled the roots in the soil until they were completely covered.