Combining 2 appleID’s

I just bought a new apple ipad. My phone I bought last year is an apple phone. My old iPad is an apple. The old iPad and phone did not connect together. They both had their own appleID. Now with my new iPad,the phone has connected together. My old iPad appleID also is on the new iPad. Unable to connect to that appleID. My question is: how can I combine both appleID’s into just one? Thank you.