Here are two sample visualization exercises to try on your own.
The vastness of the ocean
When you want to meditate, at that time think of something very vast – the sky, the ocean, the mountains – and become one with the vastness, which is all power. Imagine something very vast, calm and quiet. When you start meditating, feel that inside you is a vast ocean and that you have dived to the bottom, where it is all tranquility. If you can identify yourself with this vast ocean, with this flood of tranquility, then it will be extremely easy for you to meditate.
The sky inside your heart
Keep your eyes half open and imagine the vast sky. In the beginning try to feel that the sky is in front of you; later try to feel that you are as vast as the sky, or that you are the vast sky itself.
After a few minutes, please close your eyes and try to see and feel the sky inside your heart. Please feel that you are the universal Heart and that inside you is the sky that you meditated upon and identified yourself with. The universal Heart is infinitely vaster than the sky, so you can easily house the sky within yourself.