By a lot of experimentation with different backgrounds, different angles, different camera distances from the card, etc. I always take several photos of any cards I make and then look through each one in turn to choose the best one before uploading. Try the card at a slight angle to the light source rather than the light source being directly in front, try the camera at a slight angle to the card, which I've found to be necessary if the card has any shiny surface on it, try taking pictures from close and further away from the card. I've found that I get the best picture if I put lighter cards on a black background and realy dark cards on a white/light background. The automatic settings on my camera looks for the range of light to dark in the area of the picture being taken and tries to adjust for a range, so if you have a card that's mostly dark it will usually adjust lighter, and if a card is mostly light it will try to adjust so that the image is darker. I make sure I don't take the photo from too close, so that a wide border of the background, of whichever colour, is visible in the viewfinder around my card. And I take my pictures at high resolution. The software I use to crop and do any other adjustments after transfering the pictures to my Laptop is
IrfanView. This is free and you can crop, do automatic colour adjustments or manual adjustments, as well as do fine rotational adjustment if a card is slightly on the squiff in the photo, and finally resize down to around the 600 x 800 for the main CUP site or the 1000 pixels wide for the competition gallery.
It's mostly down to experimentation as everybody's ambient light, camera, location for taking card pictures, are all different, and of course sunlight changes during the day as well.
Hope you find out what works best for you.
GG