Creating a Custom Master Page in SharePoint 2013

 To Create a custom master page in SharePoint 2013:

1. Design your master page as html with structure folder:
   
2. Go to The Design Manage
   
3. Mapping to the Master Page Directory by Edit Master Pages:
   
   
4. Click on that link and you will see this dialogue box:
   
5. If you use  Windows Server 2012  you need to install feature “Desktop Experience” for next steps.
   
   
6. You need to select the folder containing Master Page and Page Layouts as well as their dependent assets (CSS files, scripts, images, etc.). To do this:
   
   
   • Go to Windows Explorer, right click on your computer and select Map Network Drive. 
     
     
   • Connect to network drive
   • Select url to map to folder
      
   • Now we can simply copy our template and dependent files into this folder
7. Now you can select your html master page and click insert.
   NOTE: SharePoint 2013 expects your HTML to be XML Compliant. If you have older or non-compliant HTML you will get an error. Fix your file and try again.
8. Press Conversion and successful link to see preview of this page.
   
   
9.  At the bottom of the page you will see a Yellow Box that represents the place holder where dynamic content is going to be inserted. We're going to now move that place holder into where we want it to replace our dummy text in the middle of the page.
   
10. Download the HTML file from the Share Point 2013 environment and open it in your HTML editor and you will see that Share Point 2013 has decorated your HTML with a number of tags.  These tags are used by Share Point 2013 as markup to specify where to put to insert content when it generates the Master Page. 
11. Publish HTML file it will reflect automatically in master file.