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If you do what I think you're going to do with that belt sander, you will destroy the lens. Think very carefully if you are going to do this because I'd guess that for a rank beginner with no-one to show you, your chances of doing this without major damage to the lens are less than 10%.
Practice the following first on a few magnifying glasses (you can stuff these up) before you try your objective, as you only have one chance at that and you must get it right.
How to do this properly is discussed in"Amateur Telescope Making' (Ingalls) volume 3.
Basically, you need a motor-driven vertical spindle with a cup somewhat smaller than your lens, filled with soft pitch. You also need a small light source (LED would do nicely) overhead. The speed needs to be variable, and its not particularly fast; a typical battery drill would provide enough motive power and could be cannibalised to provide the motor and speed control. You need to use a belt or similar so that the spindle is some distance from the motor as you will need to use water at the spindle, and grinding is a messy wet business; water and electrics don't mix well.
You will also need vernier calipers to measure the ID of the cell to 0.1mm, and the diameter of the lens as grinding progresses.
First step is to get the lens onto the spindle and centred.
Warming the pitch so it's soft, place the lens on it and rotate the spindle slowly while watching the reflection of the lamp off the lens. If the lens is off-centre, the reflection will wobble from side to side. When it is centred properly, it will appear stationary.
Once you have it centred, coat the lens with hard beeswax or adhesive masking tape to protect the polished surfaces from being scratched by abrasive.
Now to trim the external diameter of the lens you need abrasive. I used abrasive grits and a steel plate to make lenses for an eyepiece, however in your case you have a large lens and only need to do this once, so I'd suggest a wet-and-dry carborundum stone from a hardware store will do.
You also need to rig up a gentle stream of water flowing over the lens to wash surplus abrasive and ground glass away; a hose fed from a tap will do nicely.
In essence,
1. measure the OD of the lens, and right this down with time = 0.
2. set the spindle rotating at about 1 rev per second, turn on the water and start grinding the edge, and grind for say 10 - 30 seconds.
3. measure the diameter again and work out how much you have removed in this time. From this, predict how long you need to grind to reach the desired diameter.
4. Divide this by 3 and grind for this time, and repeat the measurements.
5. When done, use mineral turps or petrol to dissolve the pitch off the lens and clean up.
Key points to remember:
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Grind a little at a time and measure many times as you get close to the desired diameter take it very slowly - it's impossible to put glass back on if you go too far.
Keep tools, fingers, EVERYTHING off the lens surfaces. Everything will be contaminated with grit and contact with the surfaces will scratch them.
The rate at which glass grinds can be highly variable - Pyrex and fused quartz are very hard and grind very slowly. Domestic glass (borosilocate crown) is softer and grinds faster than pyrex. Many optical glasses used for lenses are quite soft and will grind quickly. The soft glasses also are both weak and brittle, applying too much force while grinding will cause it to fracture and a chip will splinter off.
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