I want to install SSL certificate for my website which is hosted on a DO VPS. There is a "free" SSL guide which I am not able to get results with.
Which Free Tutorial you are talking about ?? As there are two I guess, one Self Singed another is free from StartSSL..
Free self singed will always generate warning but the StartSSL given SSL is browser valid certificate.. If you are facing issue about the same, better to discuss in that StartSSL tutorial as comment to get further help for installation..
But there are a lot of options to choose a SSL provider. Which provider should I buy the SSL from (bigger problem) and how should I install the certificates into my VPS?
The problem actually is not from who you should buy, rather what should you buy ??
There are many category, starting down from,
DV or domain validation only.. Cheapest
Wild Card (which helps to validate every subdomain) SSL
then comes the Extended Validation for that complete Green Bar with company name on visitors browser..
Installation process may vary deepening on your back end, for example, I am running a control panel, CentOS Web Panel, so my installation process would be bit different than plain no panel apache host..
Where as expensive panels like cPanel installation can be done with few clicks..