is there a better way to ssl than plugins?

I am am hosting on siteground, they have a plugin that forces http to go via https, but one of my sites is having content going twice(This plugin shows duplicated : https://www.whynopadlock.com), once via http and once via https

is there a better way to ssl than plugins? should a manual job work better? am I supposed to buy a certificate instead of using the free one?