This is part of a post I made in an older thread. See if any of it helps you:
It's all about where you apply pressure, how much pressure, what angle the blade is to the belt and how fast the pull is.
If you have variable speed, slow your grinder down a bit. I would recommend a carbide faced file guide placed where you want your plunge lines. Folks say to grind your weak side first so that you can match it on the other side where you feel you have better control. The fact is you should be grinding both sides equally as you go. 5 or 6 passes on one side then the other, back and forth. You don't want to grind one whole side then grind the other whole side. One reason is you don't want to put uneven stresses into the blade. Grinding back and forth from side to side keeps stresses more even and it allows you to see if problems are forming while you still have the room to make adjustments. When you get to the finishing stages, do your weak side first.
Stand in front of your grinder, feet shoulder width apart. keep your elbows at your sides. Stand close enough to the machine so you can see where you are grinding. Lock your elbows in place and use your legs to shuttle your upper body side to side when making your grinding passes. With the hand that is holding the tang, place your thumb on the ricasso (or on the file guide if you are using one). The support hand will rest the spine of the knife on the forefinger and the thumb will be on the blade. We will assume that you have marked your centerlines etc. As you approach the grinder angle the blade about 45˚ to the belt and carefully lay the edge into the belt and make a smooth pull out to the tip. Repeat as necessary. What you are doing is cutting the edge thickness down to your pre heat treated thickness. Leave roughly the thickness of a quarter. Work both sides until you are there. Now you are going to start changing the angle of your blade to the belt to raise that grind up the side of the blade. You are not taking any more material at the edge, only the sides. When you put blade to belt you are gently laying the bevel you have created down against the belt, feeling for when the bevel lays flat against the belt. As you begin your pass, the thumb on the tang will add a little pressure above what we'll call neutral. Neutral is the belt removing material evenly from the top of the grind to the bottom and side to side. If the bevel you have established is laying flat on the belt, this above neutral pressure will add force to the top edge of that bevel. This shouldn't be making a new bevel or facet. This should be widening the bevel you have already made. And this is how you will progress up the blade. Add pressure with your thumb where you want to remove material. If you have a grind that is dipping a bit in the middle, put your thumb at that location. Of course the blade is upside down so if you have a dip at the spine you are putting your thumb a little lower toward the spine. As you make a smooth light pull add a little pressure with the thumb as you pass the spot with the dip. You should see the dip start to rise. Repeat until the line is fairly even then make longer neutral pressure pulls to blend everything into a smooth and level line. Adding pressure is a subtle thing. You will develop a feel for it. As your grind gets higher you will want to start forming a radius where it blends into your flats or spine. This is a balancing act of time, distance and pressure. It is also aided by running the belt off the side of the platen 3/16" or so. Lay your blade gently on the belt, slide it over to the plunge line and add pressure on the spine side. At the same time you are adding pressure you are drawing the blade away from the plunge. That radius area is a very small distance so getting a feel for how much pressure and pull speed will take some trial and error. It helps a lot to scribe the lines of your intended grinds before you start. Leave yourself room to correct. Do not grind right up to your final lines. You will be cleaning up the blade and dialing in your final dimensions after heat treatment so leave a little meat on the bone.
Here is a basic grinding video by Neels Van Den Berg:
The jig users will be along shortly.
