Can someone please explain backdoor Roth accounts like I'm 5?

Household MAGI is over 240k. How does the backdoor Roth work? I understand why someone might want to do it (tax free growth and withdrawal), but I don't understand how you actually do it. Some of my questions include:

• How much do you convert to Roth each year?
• What do you pay in taxes to do the conversion?
• What is this rule about traditional IRAs people talk about?

Thanks in advance!