The best way to secure a countertop to a bathroom cabinet or vanity is set it on the cabinet first to make sure everything looks and fits right. If so then use some pieces of blue masking tape to mark the top and bath vanity so you can put the top back in the same place. Then remove the top and use Silicone or construction adhesive like liquid nails or PL to secure it in place. Use it sparingly because it doesn’t take much to hold a top in place. Place the top back on and press it down making sure to line the tape marks up where they were on the dry fit. The silicone or adhesive will take about 24 hrs. to fully dry. Laminate tops can be done the same way except using a construction adhesive. On a wood or laminate top it is possible to use screws from the bottom but make sure to use screws that won’t come through the top.