New countertop

Last night I tried to replace the old off-white countertop in the main level bathroom with a new look one from Home Depot with a new tap.
The counter went in ok but the drain leaked a bit, I had to stop at midnight. I should have put the new drain in the sink first and then lowered it in the drain pipe. The drain pipe is forcing the sink drain off the the side making the rubber seal not fit properly.
I cut the countertop to fit on the table saw which went well.

I didn’t put those two rectangular side pieces on that fit up against the wall. I’m not sure what the point of those are but I’d have to recut the thing again. I think I’ll leave that out. Does your counter have those?