Place the sleeve on a table. Insert the first elevator and second elevator (they fit together) through opposite slats of the sleeve; drop and lock them into place at the bottom.
After you have removed the top of the soda can, push it fully into the sleeve so that it rests gently agains the second elevator's top surface. You may need to tap the can into place, as it's more difficult to insert if the can's top rim is not perfectly circular or has remnants or enters at a slight angle. The sleeve is designed to be a tight fit with the can so that it gives the can the most support as it is being worked in the sleeve.
Preferably, to inhibit the can's rotation, you should lock the can into the sleeve with one or more pushpins that pierce the can's side at the sleeve's top anchor holes. Two pushpins at opposite sides of the can work very well, since they can each bear half the pressure created during your cut. If you go the two-pushpin route, you'll want to twist the can slightly in the direction of your intended motion; then lock it in place further with the second pushpin. This keeps the can snugly in place during during the cut.
If the pushpin holes would clash with your design – or if you find using pushpins to be inconvenient or unnecessary – then you can allow the can to rotate relatively freely when needed so long as you're careful to be pressing gently down on the top of the can while cutting. It's important to keep the can from drifting upward as you use the slider, or else you will get an uneven result.
You should be dragging the flat bottom of the slider against the top rim of the sleeve as you cut. Make your cut as perpendicular to the side of the can as possible, except when you need to wiggle the slider sideways to rejigger a stalled cut.
The last bit of the cut around the circumference of the can must be done carefully, as it needs a bit more force. Alternatively, you can use scissors to take off the last bit.
As before, the last bit of the cut around the circumference of the can must be done carefully. To keep the lid from blocking the motion of the slider, you need to keep the lid of the can parallel to the ground and yet still elevated in its proper original position while maintaining light downward pressure.
If remnants remain from the cut, use the side of the metal part of the slider to crease and flatten the remnants against the top of the sleeve.
Be sure to scrape the slider along the inner rim of the can to deburr any sharp edges.