In order to be able to practice Brahmavihara, First you need to get your Metta.
How to get Metta?
First you need to be happy, if you're not happy, you won't be able to give your happy feeling to others.
Think about Happy Feeling and send those feeling to a friend
[same gender / not family/ alive] .
Try to stay with object for at least 30 mins till an hour. Then you'll realized that you don't feel your body, can't stop smiling and there's something around your head.
Then you'll send to some friends then some families then some random peoples then all of your enemies. After this you'll get your metta and you can start radiating it to your surroundings.
Once you got your metta, send it to every directions. 6 Directions as described in the suttas. Then you'll realized that you're in a very big space, with metta and an infinite space, you'll experienced karuna.
Then it changed into Mudita then Uppekha then bla bla bla until Everything Ceased.
Brahmavihara is the easiest object that even an 8 years old kid could get into arupas.