Just Copy and Paste thats all.
Regarding your issue all a user has to do is copy a form into a table in any page that references the action to the include/login.php script:
<FORM action="http://yourxcartsite.com/include/login.php" method=post name=authform>
<tr>
<td class=VertMenuItems>
<font class=VertMenuItems>Username</font>
<input type=text name=username size=16>
<font class=VertMenuItems>Password</font>
<input type=password name=password size=16>
<input type=hidden name=mode value=login>
<input type=hidden name=usertype value="C">
<input type=hidden name=redirect value="customer">
</td></tr>
<tr>
<td height=24 class=VertMenuItems>
<font class=VertMenuItems>Login </font><input type=image src='http://yourxcartsite.com/skin1/images/go_menu.gif' width=27 height=14 border=0 align=absmiddle>
</td>
</tr>
<tr>
<td height=24 nowrap class=VertMenuItems>
<font class=VertMenuItems>Register </font>http://yourxcartsite.com/skin1/images/go_menu.gif
</td>
</tr>
<tr>
<td class=VertMenuItems>
<div align=right>
If you have disabled Javascript in your browser click here
</div>
</td>
</tr>
</form>