Smart Contract to interact with an existing smart contract

I would like to develop a very simple Smart Contract that interacts between my MEW wallet and another existing Smart Contract. In particular, when my MEW wallet receives a certain amount of a given X Token (fixed), the smart contract must send them all to an existing Smart Contract interacting with it (existing function) and then after a certain time it must interact again with it (another existing function) to receive back a certain quantity of ETH. Received the ETH must send them to another wallet. I hope I was clear. I remain available for any clarification and information. Thank you in advance.