Add this to your existing aspx Page declaration:
MasterPageFile="~/SomeMasterPage.Master"
Should also mention that to add page specific content to your Page you also need to add any Content tags (defined in your Master Page) to your aspx page also:
<asp:Content ID="myContent" ContentPlaceHolderID="someContent" runat="server">
// Page Content goes here
</asp:Content>
If you don't put these in then the default content from your Master Page is used instead.
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