You should read some documentation in this direction. I know it's a bit frustrating at first to write the rules, but it gets easier. You need to learn regular expressions to write the rules (you can start here: http://www.regular-expressions.info/)
As for the rules you need, they go like this:
RewriteEngine On
RewriteRule ^signin$ index.php?req=signin [L,QSA]
RewriteRule ^useraccount$ index.php?req=useraccount [L,QSA]
or
RewriteRule ^(signin|useraccount)$ index.php?res=$1 [L,QSA]
You can paste the rules you have used, maybe someone will explain you what you did wrong.