Place your CA.crt, crt and key file here and create sysmlinks like following ...

ssl
├── proftpd.cacert.pem -> CA.crt
├── proftpd.cert.pem -> wildcard.example.com.crt
├── proftpd.key.pem -> wildcard.example.com.pem
├── CA.crt
├── wildcard.example.com.crt
└── wildcard.example.com.pem

then:
copy conf.d/tls.template to conf.d/tls.conf

finally:
uncomment '#Include  /etc/proftpd/conf.d/*.conf' in proftp.conf