go to domain -> secure your sites -> enter information and create request. use that signature you receive with comodo or some other provider to create your key.
you get 3 files in a zip from them
one is your domain certificate.
one is the root certificate and one is the intermediary certificate.
go to domain -> secure your sites -> click on your domain
in the text fields below copy the domain certificate text in there.
then open the two other certificates. combine those two into one file in a text editor. the root certificate needs to be below the intermediate certificate in the text file and then hit submit.
when no error comes up your SSL is uploaded but not activated yet.
go to websites & domains in plesk and scroll to the bottom. click on your domain
there you click “enable SSL” and under certficate choose the certificate you just uploaded. you are finished.