1) Follow the steps on https://developer.apple.com/help/account/create-certificates/create-a-certificate-signing-request to create a certificate signing request file. (Using the Mac in the office, save the certificate signing request file in the appropriate folder inside the Documents folder)
2) login to the developer account @ developer.apple.com
3) Select Certificates, Identifiers, & Profiles
4) Click the blue “+” button next to “Certificates”
5) Select the iOS Distribution option and click the blue “Continue” button
6) upload the Certificate Request File that was generated in step 1, and click the blue “Continue” button
7) double click the downloaded .cer file and in the “Add Certificates” pop up window, click the blue “Add” button to continue (In the Keychain drop-down menu, I set it to “login”)
8) Double check that you can find the newly generated certificate in your Apple developer account as well as in Keychain Access > My Certificates