You could use either a oil or waterbased poly over the stain, just make sure it is fully dried. There's no need for the shellac.

I've used a number of General Finishes products and have been happy with all of them.

Use the stain and let it dry for 24 hours. That is probably longer than the recommended drying time, but it will insure the stain is dry.

Wipe the stained wood with a cloth dampened with the thinner used for the finish product you use. If you use oil based poly, use paint thinner. All you want to do is remove any loose materials on the surface and leave a good surface for the finish.

Then apply the finish.

I build two step stools and do not use poly because it chips easily. I use Sherwin Williams non poly varnish instead. Plenty tough and more easily repairable if necessary.