To begin with, press the "New" button underneath the Fixture Library and create a "DMX Fixture" with e.g.: Vendor = _Custom, Type = Test1, Width=10, Height=10 and Offset=3.
Alternatively, you can also edit an existing fixture. Simply choose it from the Fixture Library and press "Edit". Now you can modify it as if you created a new one.
As seen left, for DMX fixtures (controlled via DMX or Art-Net), a default channel order is already applied. The fixture's top left pixel is highlighted in red because it is the first channel. From there the red line shows the DMX addressing order.
If you hover your mouse over the fixture, the bottom bar displays according pixel and fixture information including the pixel's channel ID.
If you are working with Video Tiles (controlled via DVI, HDMI or DP) the pixels are displayed as squares. Of course no DMX order needs to be displayed and no further adjustments have to be made. The ReMapping Guide might be of interest.
If the order of the channels and the other settings are alright, press the "Save" button. In case you edit an existing fixture, change the "Vendor" or "Type" name before saving if you do not like to overwrite the existing fixture.
After closing the Fixture Editor, you can find your fixture in the Fixture Library to add it to your patch.
Now, there are no addressed channels in the fixture and you can apply a custom DMX channel order.
Set the "Start ID" to 1 and check whether the "Channel Offset" and Color Mode are alright, e.g. 3 and "RGB".
Then check the option "Auto Increment ID" and draw the new order in the fixture - simply click into the fixture and drag the line.
If you made an error, you can stop drawing at any time. Click on a patched pixel to unpatch it again. If you start drawing again, the line red continues automatically. However, keep in mind, that the ID increments automatically. If you would like to "go back" some channels, uncheck the increment option, set the correct "Start ID" and check the option again.
Alternatively, you can work without the "Auto Increment" option. In that case, set the "Start ID" any time before clicking into the fixture. The red line connects the pixels automatically in the order of ascending IDs.
Remember to save changes to the fixture before closing the Fixture Editor.