Kyle you should install some sideways 2x4 or metal cross bracing under the plywood that extends from wall to wall at or near the front and again half way back of the countertop.
Install marble countertop on the wall.
You need a completely solid and stable base for the marble slab.
Record both the length and width.
Very rarely will you find walls perfectly square and straight.
They are fairly easy to install and you will be very happy with the result.
Lay the marble countertop over the wood vanity to trial fit for shape.
Measure the area in which you want to install the counter top.
Marble bathroom countertops come in various textures and color tones.
This will remove all dust.
Step 1 take proper measurements.
Make sure to immediately clean up any excessive glue from the marble countertop.
Step 7 dry time allow the countertop to set overnight after examining the installation to make sure an even consistent bond has been created.
Apply a wet wipe using alcohol run across all marble surface area before using the adhesive.
If the fit is too large you can try to cut it to the correct size yourself with a circular saw but you risk splitting the marble.
If the countertop is too large or too small contact the manufacture to adjust the fit.