1. Create a folder in the root of your MOSS web application .
2. Add this folder as a Virtual Directory to your web application using IIS.
3. Copy the .ascx files you want to use into the new folder.
4. Build your assembly and copy it to the _app_bin folder in your Web Application.
5. Add this line to your Web Application's Web.Config, within the SafeControls section
<SafeControl Src="" IncludeSubFolders="True" Safe="True" AllowRemoteDesigner="True" />
6. Add another line to your Web Application's Web.Config, within the SafeControls section, for the Assembly. (oh yeh, you gotta generate a .snk (strong name) to get the values for the PublicKeyToken=)
<compilation batch="false" debug="true">
<assemblies>
<add assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
<add assembly=", Version=1.0.0.0, Culture=neutral, PublicKeyToken=" />
</assemblies>
7. Add a page directive for the User Control you want to use, to your Page Layout or Master Page using SharePoint Designer.
<%@ Register Tagprefix="" TagName="" src="" %>
8. Now add the control within the Master Page or Page Layout
<PublishingWebControls:RichHtmlField id="content" FieldName="PublishingPageContent" runat="server"/>
<TWA: id="" runat="server" />
<WebPartPages:WebPartZone id="wpzAfterContent" runat="server" title="Content" PartChromeType="None">
Done !
From http://www.sharepointblogs.com/sezai/archive/2006/09/04/11847.aspx
