What Product to Use Over Oil Based Wood Stain?

I have stripped an old staircase and banister to bare wood, then applied Minwax (oil based stain) wood finish. The hardware store recommended using Varathane Diamond (water-based) finish over this. It has dried for 36 hours. What do you recommend from here? Is this the correct product, how should I apply, any special instructions? Research on the web shows many opinions and potential problems! Thanks.

By amateur1 from Nova Scotia