“Love G‑d and serve Him with all your heart.”
Deuteronomy 11:13.

What is a service that is performed within the heart? This could only be speaking of prayer.
Talmud Taanit 2a.

Prayer means that when your heart bursts with pain, pour it out to Him.

When it yearns, speak to Him about your yearnings.

When your heart is broken, ask Him to mend it.

When your heart feels empty, ask Him to fill it.

Wherever your heart is at, whatever it is being, connect that to His Being.

Make your heart His sanctuary, the place where you find Him.