Lay straights where the straight-aways go ... and lay curves where the curves would be.
Then sink the spline points by a little less than a meter deep into the soil (just barely visible, or slightly below).
DO NOT press the "smooth spline" tool button, until the entire grading is done, SEVERAL MONTHS FROM NOW !
As you may be changing your gradients many times before you get it exactly right. And pressing the smooth spline tool button will permanently deform the terrain.
Unfortuneately a DEM terrain can be way off by +/-20m or more in all the x-y-z areas, due to trees and schwubbery. And too the lines drawn on the terrain may have river channels way up on hillsides, and railroads and roads way down in river channels. These lines are just a rough guestimate, and can be way off by 100m in places.
On a long straight lay a ruler from one spline point to the other ... now lay another 2 rulers from the center out to each end spline point(s) ... continue doing this until you have @ 1/4 mile equal distances between all the straight track spline points. Now continuing sinking the spline points till they are just barely under the surface.
A gradient should never be over 2.00%, and should be less than 1.75% at maximum (unless it is a rare backwoods Shay line). Most of my gradients are well under 1.50%, and most often they are only 0.10% to 0.20%, with just slight leaps and dips. I try not to lay anything over 0.75% to 1.00%.
When I lay a 0.00% bottom of the grade section, then go uphill at a +0.10%, then my next will be +0.20%, followed by a +0.30% ... then a short 0.00% section, then the reverse downhill at -0.10, -0.20%, -0.30% ... etc ... etc ...
I spent hundreds of thousands of hours, grading a 4 track mainline that is only @ 45 miles long, and changed the spline point height many thousands of times, before pressing the "Smooth Spline" tool button one year later. Trust me ... Don't rush cutting the cuts, and filling the fills. You will get used to driving trains through sand dunes that block the track.
Sometimes grading a 5 mile section will be a one night breeze, only to be fouled up, and thrown way off by the next 5 mile section of track, that takes 5 days to grade.
When laying a gradient, you can type in the gradient numeral, and keep on going ahead to the next section ... but when you type in 3 gradients, fall backwards down the track, and check the the last 3 spline points were not moved by the ones ahead. And this DOES HAPPEN almost all the time !
Keep checking and re-checking, the gradients constantly, making sure they are not thrown way off.