As vegetables upon cooking loose many enzymes and nutrients effort should be made to serve them raw and fresh whenever possible.
If cooked steaming is preferable as it retains more nutrients than boiling.
Also dark colored vegetables should be chosen over lighter ones as they have a higher concentration of vitamins and minerals.
You can chop, mince the vegetables or make a puree.
You can mix the vegetables in small amounts in your cat's food.
Also remember not to overfeed vegetables to your cat and also daily feeding is not needed. For a cat not habituated to eating vegetables start slowly.
Some cats are allergic to certain vegetables. Also not all vegetables are safe for cats. So do consult a vet.
Leafy greens for dogs and cats
What are some green vegetables that I can safely feed to my cat?
Is there benefit to feeding my cats cat food that incorporates fruits and vegetables?