First, go to http://www.readability.com/ within Chrome on your iPad or iPhone and make sure you're logged in. When you are, go to http://www.readability.com/bookmarklets/.
Follow Step 1 and copy the javascript for Read Now, Read Later, or Send to Kindle bookmarklets, depending on which one you're adding.
Step 2 – Click the star icon to the right of the address bar on your iPad in order to add a bookmark. On your iPhone, tap the three horizontal lines, then the star to do the same.
Step 3 – Edit the name of the bookmarklet to Read Now, Read Later, or Send to Kindle, depending on which one you're adding.
Step 4 – Paste the Javascript into the URL textbox, then tap Save.
When you want to use the Read Now, Read Later, or Send to Kindle bookmarklet simply tap on the address bar and X out the URL of article you're adding. Then type Read Now, Read Later, or Send to Kindle depending upon which bookmarklet you'd like to use, and select it from the drop-down menu.
WARNING: Do not select the bookmarklets through your list of bookmarks. It opens a new page and will not work. You must access it via the address bar.