Once you have a Boat (It has to be a Boat, Rafts cannot go far enough west), starting from the very left edge of the main island, tap the left button 38 times (Make sure you don't hit the island right next to the main one, and if you have to avoid it, no not count the number of times you have to tap down as part of the 38). After you tap it 38 times, head south until you find an island blocked off by rocks and a smaller island. This is Lion Island. To get to the main island, go onto the smaller island and build a Bridge on the edge. The bridge allows you to cross over the water onto the main part of the island.
To enter Lion Forest, walk into the first trees on the left, as outlined in the picture. It will transport you to a Teak forest, and you will be standing on your bridge. If you walk 10 steps forward and then 9 steps to the left, you will find the only bush in the game that gives you a Purple Yam. You can retrieve a yam once every four minutes. If you continue forward instead of going to the left you will eventually find a thick copse of Teak Trees. If you follow the trees to the left you will be able to get around it to where most of the lions spawn. From here, if you continue on to the right and then south a little, you will find a pathway through the trees that will lead you into a small clearing where the rare Mushroom is.