One way to do this is to modify your file 'index.php' as follows
/**
* redirects to installation, if xoops is not installed yet
**/
include "mainfile.php";
//check if start page is defined
if ( isset($xoopsConfig['startpage']) && $xoopsConfig['startpage'] != "" && $xoopsConfig['startpage'] != "--" ) {
header('Location: '.XOOPS_URL.'/modules/'.$xoopsConfig['startpage'].'/');
exit();
} else {
$xoopsOption['show_cblock'] =1;
include "header.php";
include "footer.php";
}
Put your own content between these lines
include "header.php";
(your own content here)include "footer.php";