Think of your different relationship situations (adult-adult, and/or parent-child) and then think of how someone in a secure attachment relationship would act, and then emulate those behaviors. Repeat. Again and again.
At first it can feel weird and unnatural, but as you keep doing it, you will internalize those conscious actions into unconscious habits, and then it will feel natural. It’s kind of like writing a script for your relationships. Basically you’re trying to establish and internalize new and better habits. It’s simple in theory, but a lot of work in practice.