It's easy and becomes easier the longer that you have to do it for. You just have to adjust some of your expectations about what you can buy/eat.
I have less than £50 a week to shop for 7 people - 4 adults, 1 6ft2 16y/o boy, a teenage DD and a 10y/o. The meals I make are regularly under £3 for all of us. Tonight, I made minestrone soup - it cost £4 to make but fed us all tonight and will give me 3 lunches through the week. All that needs added is some crusty bread.
other regular things -
potato, spinach and lentil curry
pasta with caramelised onions and cheese
spanish rice with rocket and kidney beans mixed through it
chilli and rice
leftover chilli made into tamale pie
lentil soup
potato soup
pasta e ceco
chick pea and pasta soup
potato and onion pie
slow braised cabbage pasta
bean burger roils with salad and wedges
pasta with white bean sauce
patatas bravas
broccoli rice
chicken curry
chicken pasta
chicken soup
pea soup
pea dahl and rice
jacket potatoes with mushy peas, cheese tomato and onion
noodle stir fry with mandarins, mange touts, left over meat and soy sauce
Things that need separate veg get served mainly with mixed veg, salad leaves, carrot and cabbage salad, cucumber and pepper sticks, roast carrot and swede - whatever is cheap at the time. Soup is served with bread and followed by yoghurt/rice pudding
I only cook with meat once or twice a week, fish is usually tinned or occasionally frozen, except for the odd bit of smoked mackerel or packet of smoked salmon trimmings.
We eat a very varied and balanced diet, just a cheap one. When you have no choice, you learn quickly.