Welcome to the digitalocean community.
Allright so since you bought a Ubuntu droplet I believe you know how to manage a linux system. If you don't, please check out how to manage it first using WinSCP and Putty.
You can create a CSR code for an SSL certificate using putty. If you login to your droplet you can use the command in the other question to create a CSR in your current directory. So first of all go to a directory where you want to create the CSR. Use the following command for example:
If you want to create a new directory for your CSR you can use this for example:
If you create a new directory, cd into it afterwards. Now you can create a CSR in there:
openssl req -new -newkey rsa:2048 -nodes -keyout domainname.key -out domainname.csr
```means you create a 2048bits csr. I recommend for more security to use ```rsa:4069```
You also need to change ```domainname.key```and ```domainname.csr``` into your own domain name so you can verify the CSR file. After you filled in all the legit information , you can open the new file using a client for example WinSCP, copy it, and use it for your SSL.
I hope this will help you, if not you can come back here and I'll be happy to help you.