Invert the bicycle or put it in a bicycle stand to elevate the rear wheel.
Disconnect the Automatiq power harness from the hub interface by pulling towards the front of the bicycle.
Loosen the axle nuts. If the bicycle is elevated, be careful to support the wheel as you loosen the second axle nut, or else it will fall out of the frame.
The wheel will come out of the dropouts downward (and slightly forward, which loosens the belt tension. The belt will still be on the cog. Lift the belt to the right over the Automatiq interface, which frees the wheel from the bicycle entirely.
Remove the flat 17mm nut that holds the interface on.
Take the hub interface off, which exposed the splined nuts.
Hold the axle from rotating. Clamping the axle flats in a vice is easiest. You can also hold the axle with an 8mm or 5/16" wrench.
While holding the axle from spinning, rotate the outer spline nut clockwise until its tight. It needs to be snug. You can grab it with a pair of pliers but be careful not to slip or you might damage the splines. https://support.enviolo.com/hc/en-us/articles/11954303993618-How-to-replace-and-tighten-the-spline-nut-on-135mm-solid-axle-hubs
Place the hub interface over the axle, but don't engage it just yet. Adjust the angle of the hub interface so that a line passing through the flat sides of the axle line up between the O and L in the ENVIOLO logo.
Make sure the hub interface is fully seated on the hub splines.
Put the 17mm flat nut on and snug it down. Dont overtighten or it might hinder interface functionality.
To install the rear wheel, start by putting the rear wheel between the stays of the rear end. Make sure the Automatiq hub interface is on the right side and that the axles are close to the dropouts. Lift the belt over the hub interface and onto the cog.
As you push the rear wheel upwards towards the dropouts, guide the disc rotor in the gap between the brake pads. If the gap is too narrow, it means the brake lever was actuated while the wheel was off and you will need to spread the brake pads by putting a clean flathead or blade between the pads and twisting or pushing outwards. This will push the hydraulic fluid back up into the master cylinder and resent the pistons in the caliper.
After the rotor is lined up between the brake pads, the axle can be lined up into the slot of the dropouts.
The belt tension was initially set with the wheel butted up into the very top of the dropout. This makes a good baseline to get the tension proper again in the future without having to touch the sliding dropout hardware. In order to get the wheel all the way up into the top of the dropout slot, use physical weight or push down on the rear rack or saddle (tire if inverted) before tightening the axle nuts. It is helpful to have the bicycle on the ground (with axle nuts loose) and have someone sit on the rack or push down on the rack. Make sure the bicycle is perpendicular to the ground (not leaning) so that the wheel gets tightened in the frame straight.
Lastly, connect the Automatiq harness to the interface. Note the plug is shaped such that it only goes in one way correctly.
Make sure the plug is seated correctly. Push the plug until it butts up against the interface.
