DreamHost recently enabled Let’s Encrypt support. This is great and makes HTTPS accessible to a great many people. For new domains there is a simple HTTPS checkbox, could not be easier. For existing domains you need to make sure the domain’s “Web Hosting” is set to “Fully Hosted” and there are no funny redirects. If you have an Internationalized Domain Name it appears you are out of luck. If you have a great many subdomains (for which you should also enable HTTPS), beware of rate limits and wildcard certificates being unsupported.
The way DreamHost manages the rate limits is by scheduling the requests not succeeding for a week later. Coupled with the fact that Let’s Encrypt certificates are relatively short-lived this places an upper bound on the amount of subdomains you can have (likely around sixty). If you manage certificate reqeusts from Let’s Encrypt yourself you could of course share a certificate across several subdomains, thereby increasing the theoretical limit to six-thousand subdomains, but there is no way that I know of to do it this way through DreamHost.
To make sure visitors actually get on HTTPS, use this in your .htaccess for each domain (assuming you use shared hosting):
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
(As long as domains are not using rewrite rules you can in fact share this across many domains by placing it in a directory above the domains, but you will need to copy it for each domain that does use rewrite rules. IndexOptions InheritDownBefore requires Apache 2.4.8 and DreamHost sits on Apache 2.2.22, although they claim they will update this in the near future. (Very much unclear why DreamHost wiki is still without HTTPS.))
The next thing you want to do is enable HSTS by adding this to your .htaccess (first make sure all your subdomains are on HTTPS too):
Header set Strict-Transport-Security "max-age=31415926; includeSubDomains; preload" env=HTTPS
The preload directive is non-standard, but important, since once this is all up and running you want to submit your domain for HSTS preloading. You can remove the preload directive after submitting your domain, if you care for the bytes (or standards). With that done, and thanks to DreamHost’s upgraded HTTPS story, you will get an A+ on the SSL [sic] Server Test.