The syntax for the manual plugin is not all that different from other plugins. The installation instructions specific to your OS on the certbot homepage should help you get the client installed, and the simplest version of using the manual plugin is basically:
./certbot-auto certonly --manual -d example.com
example.com is your domain. The client will do its thing, and pause when it’s time to perform the domain validation, asking you to upload certain files on the old server. Once you’ve done that, the client will tell Let’s Encrypt to go ahead and check the files are present, and if that works, you get your certificates.
Were you planning on using the automatic certificate installation/configuration that’s available for apache? There might be a way to get this working with the manual plugin as well - if you’re looking for that kind of solution, I’ll try to test this and provide some instructions. Otherwise, the command above should do.