It's a long-standing tradition to paint from the Old Masters, but you should not try to pass these off as your own paintings. Similarly, 'how-to' books are there to help you develop your own skills, not to enable you to pass off the finished picture as your own original creation (afterall, you've copied someone else's composition and techniques). Make a note on the back of these paintings to remind yourself of the origins / influences. (Write on the actual canvas, not the stretching frame, so it never gets separated.)
Remember, just because a painter has been dead for many years doesn't automatically mean that their work is out of copyright; it may still be owned by a gallery or the artist's estate. Check the copyright situation, don't assume.
If you've made a painting in the style of another painter, add a note saying "After Rothko" (or whoever) to acknowledge that it's done in that's artist's characteristic style. That way you're not leaving yourself open for a critic to "denounce" you for copying another person's style at a later date. (Like Jack Vettriano was "denounced" for having used a reference photo; it was ridiculous, but it makes headlines.)
If it's a copy of another artist's painting, then add a note that makes it clear that it's a copy and not the original, so something like "After Van Gogh by Jo Bloggs". That way no-one who buys it in the future can try to pass it off as an original, which would be forgery and which might end up entangling you as the original artist. (Yes, it's unlikely, but once a painting has been sold you've no control over it.)
Some galleries and museums that allow artists to make copies of paintings in their collections by working in front of the actual painting, require such copies to be smaller than the original painting. It's another way of identifying the result as a copy.
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.