A soft-drink can facilitates a lot of creativity. It is the easiest way to obtain a piece of metal sheet which need to have the can cut open. It may not be a hard task but has the following difficulties:

1. As the can has no support inside, it is hard to estimate how hard to push the cutter.
2. The can is a cylinder that may roll and slip away when cutting it open.

In order to do it safely. This instructable shows how I have the job done.

Step 1: Tools

Prepared the following tools:
1. a cloth
2. a cutting mat
3. some blu-tack
4. a cutter (wide blade preferred)
5. a screwdriver with hard plastic handle
6. a bottle (slightly smaller than the can in diameter)
7. an old megazine
8. a pair of scissors

Step 2: Cloth Support

Find a damp cloth. stack it up into 2/3 of the can's base diameter. Place it against the can. As the cloth is damp, it helps to stop the can rolling. Only need to apply a little force on it.
If it still rolling, may also apply some blu-tack between the can and the cutting mat to make it stay still.
Suggest starting to cut near the can top first.
while holding the can still, apply force on the cutter against the can surface above the cloth. The cloth supports your wrist therefore you can hold the cutter more stable and steady. Even the can slips away, the cloth can stop hand movements and prevent cutter blade to cut anything or anyone.
To cut the can effectively, apply force gently towards the centre point of the can, i.e. move the blade back and forth along the blade. By this way the can is cut open bit by bit. Don't rush.
Roll the can a bit when the cut is close to the cloth. Continue to work until the can top is separated.

Step 3: Smoothing the Edge

The edge of the cut may not be flat and regular. Grab a screw driver on it's shaft and use the plastic handle to smooth the can edge by moving back and forth along it.

Step 4: Support by a Bottle

As the can top is removed, the can is easier to be flattened when force is applied on the open end. This is hard to hold it by hand. Also, the edge is sharp and easily cuts.
Find a bottle slightly smaller than the can, wrap it by an old magazine and place it inside the can.This forms a support inside the can that avoids deformation.