Do some searching (maybe elsewhere?) for "copy work" . ..
The basics - tripod, film matched to your light source, enough lights to give your meter even readings all over the work (if they are any larger than 2.5' on a side, i'd recomend at least 4 lights- say if the work is 2'x3' . . . ), light meter (onboard is fine), and polarizing gels.
The "by the book" model is that you set the lights up at 45 degres to the surface, but in practive, you will have to play with the angles, and the distance. I've found that copying my 16"x20" prints requires sharper angles than 45, and the larger the work, the sharper the angles... I dont know this for a fact, but it seems to me is that the 45's should be in relation to the edge of the work, not the center (again, particularly larger works). If you set the lights at 45deg to the middle of a large canvas, the angles to the edges will begin to grow towards 90's.... draw it out, it will make sence to you on paper...
Last item that I'd consider indespensable - polarizing filters. Idealy, you'd have polarizing gels over each light, all oriented the same way, and then one on the lens, perpendicular... I've used a simple circular polarizing filter on the lens alone with limited sucess.
What's crucial is that you get even lighting-- I use an incident meter, reading all along the edges of the work, or a spot meter and read a blank sheet of paper in place where the work to be copied will be.. check everywhere: corners, mid edges, center -- and once you have even light, and the piece is in place, from right where the camera goes, look for spectral reflections bouncing off the surface of the print.
Oh... and if this is a one shot paying gig, do some practice work on your own, before hand.... if that's possible.
What I wrote makes it all seem more complicated than it is.. sorry.. it's not that big a deal... just watch the glare, get even lighting, and keep the camera dead center on the work.
Oh. . . and be aware of what focal length you must use to avoid any barrel/pincushion distortion. As the work is surely square, it's edges will certainly show any distortion in relation to the square edges of the image.