Dead easy. First look in your fridge, freezer and cupboards and see what you already have in. Then spend half an hour or so with a drink, a notepad and pen and as many recipe books as you like!
Then make sure the meals are balanced throughout the week. So have different sources of protein, carbs etc. That might mean pasta once, rice once, potatoes twice (jacket one night, mash another?), couscous etc. Look at ways you can make one sort of meal go two or three times in different ways - roast chicken, then later in the week have cold chicken salad or chicken tortillas, then use the carcass for stew or soup (or freeze to do this next week if you're sick of chicken!).
Then make your shopping list, making sure you account for breakfasts, lunches and all the staple stuff - flour, sugar, tea or whatever. I always make sure I have in things that can make a quick meal if I need to change my menu-planning for whatever reason - for example, I might have planned a meal that takes a bit of 'looking after', then find we have an unexpected event crop us, so it'll be a quick soup or pasta and tomato sauce or an omelette or even beans on toast!
Plan your meals around your regular activities - if you know Mondays are hectic because of kids' swimming lessons or whatever, make sure Monday's tea is dead easy or can sit in the slow cooker. Save the more time-consuming meals for when you have time!
Also, be prepared to be flexible. If I have more leftovers than I expected (perhaps one child was unexpectedly out to tea), I will either freeze them (if they freeze well), or I'll make them form the basis of a meal for the next night, and just defer the planned next night's meal. So for example, yesterday we had a bean salad, and there was quite a bit left. So I cooked up some rice and stirred the beans and some veggies into the rice, with a few marinated mushrooms for good luck, and hey presto today we had a delicious rice salad. We'll now have the meal I'd planned for Monday on Tuesday instead!