Download the desktop client form the download section. Simply run the aplication. After a while you should be presented with the following view:
Press the "File" buttom and select appropriate file. Currently undoo supports GIMP's .xcf file format.
From now on every modification of the file is tracked. Tree structure allows you to revert to any previous version. To do so: select the version and press "SWITCH TO THIS VERSION" bottom (located below the image showcase). Then, reopen the file in GIMP.
For better visualisation of differences between two versions, undoo allows to choose among several options, such as: "Side by side", "Before / after", "Onion skin".
If "Commit on demand" is turned on only by pressing the "Commit" bottom the program will store current file's version.