One of the worst ways you can tell them is by telling them. I am assuming you’re talking about your wife—don’t just go and tell her that she cooks badly. You’re not only going to hurt her but also will create problems for yourself. (If you know what I mean ;))
You can suggest them indirectly and subtlety. Like instead of saying “you cook badly”, say “the food doesn’t tastes well today. Seems like it lacks your love!!” If you do this every once in a while, the other person will definitely (and hopefully) pick up the clues and ask you directly about her cooking skills. Now when she asks you this, don’t get carried away with your emotions. Restrain yourself and answer he thoughtfully.
Don’t say “no, never, you don’t know how to cook”. Instead, say “sometimes it doesn’t taste good. You make very good curry. But your Biryani doesn’t always taste good.” This way, she will also learn where she lacks, and she will not get hurt.
Also, if you want her to improve, don’t order her to learn. It won’t help and it would make her feel bad. Instead, you must aim to work along with her, in helping her improve by being her side through the cooking process. Start cooking some food yourself. Also take cooking classes together. Both will learn and will have loads of fun in this learning process.
Any OK cook will set up their mise en place before starting to cook. In the event that there's something missing they'll know it, at that point, and can be prepared for a substitute or a snappy outing to the market.
Mise en place encourages the cook to prepare the fixings and all together so there's no running about the kitchen or rearranging about in the wash room with a hot skillet on the oven unattended.