How to rewrite a URL using htaccess?

Question

I have read a few articles on rewriting using .htaccess and I still cannot fully get the outcome I was hoping for.

I have a url called: sitename.co.nf/Profile.php?u=Admin

Admin is my username. I want to know how to rewrite the url as sitename.co.nf/$username

$username is a variable I have set up in my PHP code.

I tried using this rewrite without luck RewriteRule ^([a-zA-Z0-9_\-]+)/?$ Profile.php?u=$1

Here is some other userful code you may want to see...

if (isset($_GET['u'])) {
    $username = mysqli_real_escape_string($connect, $_GET['u']);
    if (ctype_alnum($username)) {
    //check user exists
    $check = mysqli_query($connect, "SELECT username FROM users WHERE username='$username'");
    if (mysqli_num_rows($check)===1) {
    $get = mysqli_fetch_assoc($check);
    $username = $get['username'];


    }