1. Find some bamboo. It spawns in Jungle Biomes, but rarely will it spawn in a single single shoot. It's most commonly found in a "Bamboo Jungle", a new variant inside a regular jungle. I was struggling to find one when I went out looking.
To make it easier, when creating a new world, click the arrow next to the spot to type in a seed. There you can pick "Jungle Temple", and for that world you'll spawn in near one. There is a good chance bamboo will be in the chests inside. If you need to grow more, they grow just like sugar cane.
2. Find pandas. They are located in jungles walking slowly on the ground. They are not hostile.
If you hold bamboo out, they will follow you like other animals.
3. If you give the pandas bamboo, they'll just eat it and there won't be any hearts. To breed, pandas need at least 8 bamboo blocks within a 5 block radius. So just grow a bunch, encase the pandas in a love room of bamboo, feed both, and a baby should pop out shortly.