Varnishing

This lady will help you.

https://www.youtube.com/watch?v=QnEvYiH7Adw

Use a foam brush rather than a bristle brush. I also prefer a water based polyurethane. Varnish and oil based poly give a yellow tinge finish. Varnish tends to yellow with age.

Rub down between thin coats, and dont press down on brush - Glide

You may be using the wrong kind of brush. Check with your supplier for the correct brush. And then use a light hand.

Instead of using a brush use a lint free cloth and dip in your stain and apply. This will give you a nice line free finish. Also use extra fine steel wool and sand between coats.

Sand until smooth, think coat of stain or paint, sand, thin coat of stain or paint, sand. sealer or polyurethane and sand until smooth. Another coat of sealer or polyurethane and sand.

a smooth finish on furniture or any wood is all about sanding.

Be sure to use light strokes and as few as possible. I've had best luck with a high quality natural bristle brush, foam brushes often leave air bubbles. A light touch is the key.