First register on
http://www.instapaper.com/user/register - it only takes 5 seconds. Enter a user name, a password is not needed at this time, but it is highly recommended to set a password later to protect your account.
When you are on a page that you want to read later left click the toolbar button
or right click page > Extra Services > Read Later > Save the current page.
When you ARE logged in your account a "Saved!" message will briefly appear in the left upper corner of the page.
If you are NOT logged in your instapaper account you will be redirected to
http://www.instapaper.com/user/login
After you logged in the page gets saved automatically. No more user action is needed.
With an instapaper account you can edit/delete saved pages, create folders and more.