How to feed parakeets fruits and vegetables?

Anything leafy can be attached to the cage with a clothespin clip.  "Meaty" pieces can be offered in a small container, like a jar lid.  Don't leave any fresh food out for more than a few hours.  

Cut the items up so that they are small and put them in their food bowl where they expect food to be.

you have to try different ways. If the birds aren't used to eating those things it can be a long time before they will try them out. You have to be patient, and persistent. You will probably have better success with vegetables. Corn is a favorite usually. If you get frozen corn let it thaw completely and then put it in a shallow dish, preferably a dish the birds are already familiar with. You can dot this with frozen mixed vegetables too. Parakeets love to "bathe" in wet greens. I use nice big lettuce leaves. I rinse them in cold water and lay them flat on the floor of the cage. The birds roll around and flap their wings in the leaves to bathe. Later they will nibble on the leaves. You can shred raw carrots, see if they accept them that way. Try chopping them fine too. Sometimes the way the food is presented is the deciding factor to eat or not to eat. You can make your own wheat or bran muffins with lots of extra added egg. My birds love those. You can hard boil an egg, mash it fine, and offer that, a small amount at a time. Very good protein. Never leave egg in the cage for more than a few hours, as it goes bad. The veggies my keets love the most are corn, raw broccolli tops and carrot tops (greens). Remember that if you have birds that aren't used to eating these healthy foods, you must have patience and keep offering them, day after day. There will be a lot of wastage, but with persistence you should succeed. Another excellent food is seeding grasses from outside, but be sure the area is not sprayed with anything first. You can sprout your own keet seed too, sprouted seed is highly nutritious, just look up how to sprout seed for birds. Good luck!

The only fruits I have are my brother Ben and my uncle John and the only vegetables I have are my grandparents, so I guess how I'd do it is by using a food processer, like a Cuisinart.