I've had the same experience.
It is better to ignore the heartbeat and focus on the breath for countless reasons!
Calmer mind, more stable heartrate, blissful, easier, wind element cultivation... the reasons are endless.
There are no good reasons to meditate on the heartbeat except until you have attained the 3rd jhana at least, after which you can practice extreme ascetic practices.
But you asked for how to meditate on the heartbeat:
all meditation methods are the same, notice the in and out.
Gradually you will notice the space between the in and out.
Include this space in your observation: in, space, out, space, in, space, out, etc.
Gradually this space will open up and you can enter jhana.
For more info on meditation and how it works read this.