Seems you are not aware of the master page and the content page work flow. If you want to design anything on your content page it is simple as you are designing other pages.
Below i have placed a textbox in the content page.
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" CodeFile="testPage.aspx.cs" Inherits="testPage" %>
<asp:content id="Content1" contentplaceholderid="HeadContent" runat="Server">
</asp:content>
<asp:content id="Content2" contentplaceholderid="MainContent" runat="Server">
<asp:textbox id="txtID" runat="server"></asp:textbox>
</asp:content>
hope it helps you :)