http://monergiwmuoskoc43fj6vfbcrll5wjrbl647cdgg2zuwkbu77o7v5vyd.onion/accepting-monero/overview.html
You need to run a node for each asset you wish to accept. There are exchange plugins which allow users to spend other assets, and you will receive the asset of your choice, such as Monero. You will need to set up your own server, such as a VPS.