The basic idea is to find somebody else who is willing to pay money to obtain dogecoin. This is difficult to do by yourself, so there is the idea of an "exchange" where people who are interested in buying and selling can find each other.

You would create an account on an exchange, send your freshly mined dogecoin to your exchange account, and then put them up for sale at some price. Hopefully, another person who wants to buy dogecoin will meet your price. When the sale happens you would get some amount of dollars in your exchange account. Note that in order to sell your dogecoin, there must be another buyer who is willing to buy them. If nobody is willing to buy, you can't convert them to cash.

Finally, after a successful sale, you arrange with the exchange to withdraw your money into your bank account.