Yoast makes submitting you sitemap very easy by using the same URL for every plugin that Yoast has.

Open Google Search Console, you should already have your website added before you try to add a sitemap.

Select your website, look on the left side of the page and click on 'Crawl' and then 'Sitemap'. Remove all old sitemaps that are listed for that website.

Click on 'ADD/TEST SITEMAP' and put ' sitemap_index.xml ' into the box next to your websites name, then click 'Submit". At first it will say pending but don't worry it does not take long for it to start working.

Every time you wright a new post/page or make an update Yoast will automatically ping the search engines to have them come crawl the sitemap.

Click next to learn more about sitemaps.