How to correctly write a redirect in .htaccess mod_rewrite?

Question

Apache HTTP Server

Actually, I am not very familiar with mod_rewrite, such a task:
You need to change the page addresses www.domain.com/index.php?mod=smth to www.domain.com/smth/
I read the manuals, I try

RewriteEngine on
RewriteRule ^/([a-z]+)$ index.php?mod=$1 [R=301,L]

Doesn't
work Tried with query string

RewriteCond %{QUERY_STRING} ^mod=(.*)$    [NC]
RewriteRule ^/$? /%1      [NC,L,R=301]

Gives a 500 error
Tell me where to dig?