The easiest way would be to add new a new page with the masterpage already set then copy what you have done in the original page into the content control.
If you want to do it manually then you have to make sure the masterpagefile is list in the page tag ie:
MasterPageFile="masterpage url"
Then add the content control with the corresponding ContentPlaceHolderID:
<asp:content id="Content2" contentplaceholderid="ContentPlaceHolder1" runat="Server" xmlns:asp="#unknown"></asp:content>br mode="hold" />Then cut/paste your pages html into that... Note that all head and body html components should be removed as they are already covered in the master page.
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