You would edit the .htaccess file using the cPanel File Manager or using an editor on your computer and then upload it to the server.
You should include the following content in top top most area of the .htaccess file. You would replace the xxx.xxx.xxx.xxx with your IP address. If you are unsure of your own IP address then you could look it up by browsing to the following: http://cloud.webpal.net/information/getip.php
<Files ~ "^wp-login.php">
order deny,allow
deny from all
allow from 24.224.141.162
satisfy all
</Files>
ErrorDocument 403 "Not acceptable"
To give you an example of a complete typical Wordpress .htaccess file please see the following:
<Files ~ "^wp-login.php">
order deny,allow
deny from all
allow from 24.224.141.162
satisfy all
</Files>
ErrorDocument 403 "Not acceptable"
RewriteEngine On
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]