To get the Slime Key, you need to go into Kanalet Castle and collect all the Golden Leaves. Once you complete both these things, you then need to go to Mabe Village and get yourself a Shovel. You can buy this item from the shop for 200 Rupees.
How To Get The Slime Key
When you have the Shovel, return to Richard’s house and speak with him. He thanks you for getting his Golden Leaves and instructs you to push a statue. Doing so opens a passage underground.
Head down and be careful, green blob enemies will come up from the ground. Push the stone out of the way and jump the hole to get at the chest. Then go the other route and take the stairs up.
This route takes you out in the field outside Richard’s house. What you want to do is cut your way through the bushes, but be careful. Many of them hide potholes, so don’t fall in. Slowly make your way south, the southeast, the northeast, then northwest. You are trying to make your way to the statue in the top left corner of the field.
Once you reach the statue, dig at the foot of it to find the Slime Key. From here you can jump across a hold to the left, then use the key.
You won’t be able to enter the dungeon just yet. Head south, all around Richard’s house and field, until you come to a gap you walk through. Head south to the small islands, and then jump between them to reach the door of the dungeon.