First you need to show the bookmarks/favourites bar in your browser. Here's how:
With your mouse, drag this Link Slack Team bookmarklet to your bookmarks bar, where it will appear as a button.
Next, navigate to one of your Slack teams using the same browser.
Finally, click on the new 'Link Slack Team' button in your bookmarks bar. This will link your team with our client and you only need to do this once for each team you want to link. After that you can use the 'Login' link (top-right) on the Stacktodo website.