Bathroom sink drain assembly is not as difficult as it seems. Since you will not be canceling any wall or cutting into existing water pipes, the risk of flooding your home is minimal. It is the perfect project for the first time to do things themselves. Cover the inside of the drain hole with silicone sealant. Put the sealant only in the depression, where the drain hose will sit.
And then keep the rest of the drain assembly in place under the sink. Make sure that the lever where the battery plug adheres is on the back of the sink, below the plunger plug hole. Have an assistant screw in the drain hose. Tighten the large nut under the drain. Pressure should be applied to the rubber gasket perfectly, but not too tight. Clean any sealant that squeezes around the edges of the drain hose in the sink.
Attach the P-trap to the drain assembly and the water outlet of the fitting. Tighten the sliding nuts. Then connect the lever stop drain assembly between the taps. Allow the sealant around the drain sleeve several hours to dry. Close the faucet and run the water in the sink. Check under the sink for leaks. And some plumbers will tell you to use plumber’s putty to seal the drain hose. Silicone sealants will give you a longer life and more adhesive seal.