To create a .pem file, is quick once you have your .p12 certificate. I won't go through the process regarding p12 file generation as there are many sites that outline these steps. If you cannot find a site that outlines this make a comment below and I'll point in the right direction.
Open the terminal and go to the path where you save the apns-cert.p12. where and follow the below command to generate the .PEM file.
The command below depends on the following two elements: