Not exactly a tutorial, but it is well commented.
#### RETURN EMAIL ADDRESS ####
if($_POST['from']==''){$log .= 'Must provide a return email address.';
}elseif (!ereg("^[^@]{1,64}@[^@]{1,255}$", $_POST['from'])){
$log .= 'The return email address is not a valid pattern.';
};
#### FROM NAME ####
if($_POST['fromName']==''){$log .= 'Please provide your name.';}else{
$name=explode(' ',$_POST['fromName']);
if(count($name) < '2'){$log .= 'Please provide your given name <span style="text-decoration:underline;font-weight:bolder;">and</span> surname.';};
};
#### SUBJECT ####
if(trim($_POST['subject'])==''){
$log .= 'No subject.';
};
if(trim(strlen($_POST['subject']))>255){
$log .= 'Subject exceeds maximum length of 255 characters. Please use the form provided.';
};
#### CONTENT ####
if(trim($_POST['content'])==''){$log .= 'No content!';};
$length = strlen($_POST['content']);
if($length>4000){$log .= 'Your content is too long: '.$length.' characters!!';};
#### MULTIPLE SUBMISSIONS ####
if(!prevent_multi_submit('mailForm','')){
$log.='You cannot submit the same form twice!';
}