D3Bit has very minimal UI and it's functionality is very self-explanatory. You can just keep it running in the background when you are playing. When you need to scan a tooltip, just hover over that item and press F12 (re-bindable). Parsed out stats will be shown in an item card where you have extra controls for editing, uploading and comparing the item.
Currently, D3Bit has 2 major features: Tooltip uploading and item comparison/analysis using d3up.
The Upload function can be found after clicking on the upload button on the bottom of the item card. It lets you upload the tooltip to either D3Bit.com or Imgur.com.
For the comparison to work, you'll be supplying D3Bit your D3UP username. After that, you can click the "R" button to load your existing builds into the select box. Once you pick out a build (to compare against), you'll be all set. The Comparison function can be found after clicking on the Analyze button on the bottom of the item card. It'll bring you to a D3UP item comparison page directly in the item card.
From there you'll get to see, among other things, the differences in stats compared to the build you've chosen from the client's main GUI. Your recently compared items will be kept in a section to the right of your "My Items" page on d3up.com. From there, you can copy each compared item to your My Items list if you wish to use them in your custom builds, etc.
Note: To manually add/edit scannable affixes, please edit data/affixes.eng.json. See the Localization section for more detail on this.