Submit sitemap.xml file. Make sure that all URLs in sitemap redirecting to https version. Add https version sitemap.xml location in robots.txt. Then google automatically index your website .

Make sure that on your dashboard you create and verify each and every version of your site so you can keep track of url errors, links etc. So rather than looking to change the http version you just add the https version.

Go through the usual processes for the individual site verification, in this case then https:// version.

Redirect all subdomains to the https:// version.

There are people who see having a site wide https as unnecessary, and believe there is an issue with regards to link juice not being passed on properly. This is contrary to any of my experiences. I have several sites fully https: and rank very well.

As others have indicated you do need to do some redirects. Changing to https has been kno wn to have some seriois issues if not done correctly

Google has a short guide here:
https://support.google.com/webmasters/answer/83106?hl=en

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In addition to changing all website URLS from HTTP to HTTPS, You would want to 301 your website from the http to https. This is very important for transferring any existing link power you might have for the website.

Here is a decent code sample for Apache I found.

Make sure to keep a copy of the .htaccess if your new to this, if you make a mistake the website will be down until you put a working copy of .htaccess back up.

There is a decent guide here for apache or other servers:
https://th.godaddy.com/help/redirect...matically-8828

First, 301 redirect all http urls to https urls respectively.
Second, verify https website separately in Google webmaster (now search console).
Third submit new https sitemap.
And sit back and relax. Check how urls de-indexed from old sitemap and indexed from new sitemap. Check traffic and search analytics data to track ranking and see if there is problem in any section of your website.

So there would come a period when there would be crawled two http and https links and considered as duplicate? Or not?

If you 301, nothing will be considered duplicate.

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Yes the first thing you do is use a 301 redirect from the old http to new https domain.

Next, add the https domain as a new property on Google Search Console (if you have both non-www. and www. domains, add both, and then set one of them as the preferred domain under Site Settings).

Lastly, submit the sitemap to the (preferred) domain via Search Console. You can use Xenu Link Sleuth to generate a sitemap of your https domain for free, but the Yoast plugin does a pretty fine job too.

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After redirecting http url of your website (which are remain to update as https) to https you must have to update those urls in your sitemap.xml and resubmit updated sitemap in webmaster tool to let google know.