help with a plugin

Following the instructions for the avatar plugin install, how do I:

2) Add the following to your .htaccess under the line that handles uploaded files:

RewriteRule ^(.*/)?avatar/(.*) wp-content/avatar.php?file=$2 [L]

3) Create the following directories and chmod them to 777:

/wp-content/avatars/user

/wp-content/avatars/blog

I think I found my htaccess in my www folder, but I am reluctant to open it before I know what I am doing. I guess I can open it with notepad but I don’t know if I’ll be able to identify which line handles uploaded files. Also I don’t know how to create a directory, but I think I could figure it out. Sorry for what seems such a remedial question up on these boards, but I’m really new at this.

Thanks,

Chad