Typically I only do this if I need to run a server that uses PEM’s only.
Convert CRT to PEM
openssl x509 -in SOURCE.crt -out SOURCE.crt.der -outform DER
This will make a DER file that can be removed afterwards.
openssl x509 -in SOURCE.crt.der -inform DER -out SOURCE.crt.pem -outform PEM
This will ultimately create the PEM file for you. You can safely delete SOURCE.crt.der 🙂
Convert KEY to PEM
openssl rsa -in SOURCE.key -text > SOURCE.key.pem
This will create the PEM file from your key.