Make them easy, obvious, attractive, and satisfying.
Strip down and simplify the habit you are trying to build, and start small. Too greater difficulty, or too much too soon, will make it both difficult and unattractive. As James Clear describes in his book 'Atomic Habits', small incremental changes compound - "Tiny habits, remarkable changes".
To make a habit obvious, you can design your environment to prompt the habit - and this also works for habits that you're trying to stop. If you are trying to drink more water, leave a jug of water and a glass on the kitchen table. Trying to eat more fruit? An attractive-looking fruit bowl in plain sight will go a long way, for no extra effort. In the same way, if you're trying to cut down on say, sugary cereal, opening up your cupboard to a selection of cereal boxes under your nose is setting yourself up to fail. Create queues for habits you want to build, remove queues for habits you want to drop.
Reduce any kind of barrier or friction to the habit you want to build.
Finally, make it satisfying. "What is immediately rewarded is repeated", again by James Clear. To really build a habit, we must work with the grain of human nature, which seeks instant gratification. By adding gratification to habits you want to build, you give yourself a feeling of success at that moment. There's just one rule: Ensure that the reward isn't conflicting with your goal. Following up an apple with a chocolate bar defeats to object of the habit you're building. If you're trying to save money for something in particular, why not open up a savings account, and every time you opt against a purchase, you instead transfer that value into the savings account. Here you're providing instant gratification of seeing the savings account grow, for what otherwise might have been an unattractive decision to make.