Here I will show how to convert a HTML file and create a Master page in SharePoint 2013. This functionality is not available in SharePoint 2010.

Open a SharePoint site and click the GEAR icon.

Now go to Site settings.

select site setting

Then click on Master page and the Page layout link.

site setting

Create a folder inside it and insert all the design files with the HTML file.

master page

Now go to the design manager.

go to design manager

Click to open the design manager.

design manager page

Go to the edit master page from the left navigation bar. It will open a model dialog. From that page choose the folder of where you uploaded the designed file earlier.

uploaded

Now select HTML file and click on the insert button to convert it. It will redirect you to the Design Manager page and will show the conversion successful message.

index

design manager

Now if you click on the index page it will open your converted file as in the following figure (in my case).

change preview page

Thanks, I will write about additional issues in a while about HTML page conversions into a Master page in a few days.