Based on your question, you're on the right track. Taking each item:

• Lighting: You'll generally want even diffused light across the entire painting. Unless you have a bit of experience with flash, this is easier to set up with continuous lighting because you can see the light as you make adjustments.

• Composition: Artwork is generally photographed head-on with the camera in line with the center of the piece.

• White balance: as you note, setting the white balance for your shooting environment will ensure accurate representations. The "white round thing" you note is usually called a white balance target. WhiBal makes a few popular targets, and the XRite Color Checker Passport is also seeing widespread use.

• Other: one thing to keep in mind as any subject is photographed from a wide angle is that there will likely be some lens distortion. On a rectangular piece of artwork this would be most noticeable in that the edges of the piece (which are straight lines) might appear to curve a bit as one reaches the edge of the photo. There are a couple possible ways to mitigate this effect:

  • Use software which can correct the lens distortion. This can be done manually in various programs or automatically with Lightroom's lens correction feature if a profile exists for the lens in use.
  • Use a tilt-shift lens which allows for the correction to be 'dialed in' at the time of the shoot. Tilt-shift lenses are generally quite expensive and have a learning curve, but they're specifically designed for these sorts of situations.