For these instructions to work you will need to have generated your CSR For your certificate and have a pending request waiting on the website.
The First step you will need to do is follow the instructions on the article below and backup the pending request.
Once this is done you can the use your CSR you have generated previously to order and install a temporary or trial certificate onto your server.
Once you have this temporary certificate installed and working you will need to follow the instructions on the below URL to restore your pending request.
Once this is done this will then allow you to install your full certificate the normally once it arrives using the "execute pending request" option through IIS.