First thing to note with editing the master page on SP 2013 is that it isn’t the master file you’ll be editing it’s the html element of the master page. Creating your own from scratch is simple to do if you know HTML and CSS. Just create your html as you would a web site and then upload it, Design manager gives you the code snippets to add the many SharePoint controls and then all you’ll need is to publish it.
One issue you’ll probably find and which is a know issue is the editing of the oslo.html file will break the add an app and change the look pages. Luckily there is a fix for this which is reasonably simple just by setting these place holders to visible=”true” it’ll work again, that’s if they are set to false in the first place which you probably find is the case. Place holders that need changing are DeltaPlaceHolderLeftNavBar, PlaceHolderLeftNavBar, DeltaPlaceHolderPageTitleInTitleArea, and PlaceHolderPageTitleInTitleArea.