Htaccess Redirect to Secure Https ?

Hey guys, I have been reading and trying all sorts of things I find about how to do this successfully.

I have a subdomain with installed SSL and using this for checkout when customers buy something, so there isn’t any content or anything other than a checkout.

I just want to ensure all traffic goes to the https and not the http.

I tried the really simple plugin but couldn’t get that to work.

thanks for any help!