Remove the top cabinet drawers from the island cabinets.
This will make it easier to smooth the caulk bead between the countertop underside / cabinet seam and to clean any excess caulking.
Apply a bead of silicone to the top outer edges of the island cabinets.
Lift your island countertop slab and rest on the top of the cabinets. Adjust the position of the island countertop slab on the cabinets as needed.
Use your finger to smooth the caulk between the countertop underside / cabinet seam and then use a wet rag to clean any excess caulk.