I don't know if there is a "proper" way to layout step rails. I do it several way and I am sure others do it other ways.
Concerning your current situation. If your fir post are plumb and when you insert your rail between them do your Balusters appear vertical with a level, then your pitch is off. You have to play with them to get it right or build again. Otherwise, the post might be not be plumb.
They way I lay out steps: on short runs (3-4 ft) I will take the pitch directly off the steps. I measure the rise and run of a couple of steps estimate an average I transfer the measurement to a large framing square that has brass stops on it.
On long steps I use a long protractor. I fabricated from 1/4x1-1/4in flat bar. Lay one leg of the protractor across the steps the other vertical (level) tighten the bolt. Save the measurement. This works though be aware that different sides of the same steps might not have the same pitch.
I used to measure each riser and tread of each step with a level and step measure then draw them on a table. This is good, saves material, though time consuming. It is great for laying out post and determining where they will lay each step. If you can, then I think this is the best way to lay out a step especially starting out.
I never use a angle finder. I started out using them, kept breaking them. Also, I build rail laying flat on a table. Those fractional degrees over long run make a difference. I have never tried a digital angle finder.
Having a degree in mathematics I thought using trigonometric function was they way to go. Though trig has it place, I prefer to do everything practically.
Finally, when I begin to build the rail I determine the spacing between the top and bottom rail. To determine spacing, I insert a couple balusters at correct pitch between the rails, then make sure the rails are parallel by measuring. When the rails are parallel I will brace them between angle. Tack the angle to the table and clamp the rails to the angle. From there all the cut pickets fall into place.
www.nomma.org sells videos on rail building and they used to have some free information online about building rails.