Score the card stock first and be sure to work with the grain of the paper: folding one way will give you arcing pages, the other will lay much flatter.
The best tool for this is called a bone folder. It will not tear the paper fibers so your score is much less likely to fray.
As for the artwork, that paper has a grain as well but it may not be as evident. Try the same test and print your artwork accordingly, if possible.
When you mount the art, be very careful to apply your adhesive evenly and completely. I've had the best luck waiting until the adhesive has set to complete the fold. A few hours under pressure (heavy books) or, if you have the time, overnight will do.