You have 3 Tables at your disposal, and pressing the right buttons copies or moves cards between them.
To actually see some cards, you need to do one of the following:
Once you're happy with your cards you can save them into a deck by setting a name and clicking the "deck named" button.
Decks should be saved automatically in your browser and re-loaded at your next session; but if you really value them, you should export them and paste them to your favourite text editor and save them to your disk (having the "export for xmage" checked will export the specific card from a given set; having it unchecked will only export its name).
Beware that there is no undo implemented yet!
Keybindings which are not shown on screen:
You can sort the Tables by multiple fields. To sort by a single column, click its header (twice to sort descending). To select secondary and further columns to sort by, click their headers with the Shift button after choosing the primary sort column.
You can also select multiple cards at once by clicking the first card, then clicking the final one with Shift. All the cards between these will be selected.
This editor is open-source and you can find its source on GitHub.
Uses booster generation code from mtgen and card data and images from Scryfall.