How do you glide backward on the outside edge? Turn the head back, shoulders on the circle, skating knee bent and hold the pressure on the outside edge.
The honest answer first: in the backward outside edge you glide backward on one foot, the blade stands on the outside edge and you follow a half circle. The Learn to Skate USA leaflet describes for the backward half-swizzle pumps exactly the posture you need: shoulders and arms follow the circle line, the head is turned back into the circle so that you can see where you go, and the gliding foot stays on a strong backward outside edge [1].
In the Learn to Skate USA adult curriculum, the backward outside edge and the backward inside edge on the circle, right and left, are on level 4 of 6 [2]. Before them come backward skating, backward swizzles and the backward one-foot glide. We therefore classify the skill as "intermediate".
Honestly: the leaflet describes only briefly how the feel of the edge arises. Everything else is practical experience; a coach is especially valuable here because going backward you cannot see what your feet do.
Stand on the circle, shoulders and arms following the circle line, the head turned back into the circle so that you overlook it [1]. This is the starting posture that the leaflet names for the backward half-swizzle. Milestone: you stand such that you can see the circle line without holding on.
The leaflet describes the pumps like this: one foot is the gliding foot, the other the pumping foot that pushes out in a C motion and then draws back to the heel of the gliding foot [1]. Practise this pumping with a secure gliding foot.
The gliding foot stays on a strong backward outside edge [1]. Bend the skating leg clearly and let the edge carry you instead of steering it with the foot. Milestone: you glide a short half circle backward on one foot without putting down.
The free foot lies close to the skating leg; it does not swing out. Shoulders and arms stay on the circle so that the upper body does not turn against the curve [1].
The curriculum asks for right and left [2]. Extend the arc counting, one count after the other.
Your progress shows in the track behind you (rule of thumb, not a study): turn around after the arc and see whether the line is evenly curved. Do you manage a half circle without the head tipping forward or the shoulders dropping out of the circle? When that works on both sides, you have the basis for backward turns and later jumps. A phone video from behind helps more than any gut feeling.
This guide explains and motivates; it does not replace supervised training. Going backward you do not see what is coming: check the lane beforehand, stay at the edge and wear a helmet. In a video analysis of 216 ice-skating falls in children, 93% of the children tried to break the fall with arms or hands, and in 13% the head hit the ice [3]. Do not fall backward onto the tailbone but sideways onto a hip, knees bent and chin tucked [1]. Do not practise at peak times.
Practise the backward arc as a track on the ice: glide an arc and then turn around. Look at your track. A clean outside edge draws an evenly curved line, a juddering track shows you where you left the edge (practical experience).
A glide backward on the outside edge of a blade that carries you into a half circle. Shoulders and arms follow the circle line, the head is turned into the circle [1].
In the Learn to Skate USA adult curriculum it is on level 4 of 6, right and left [2]. It is preceded by backward skating and backward glides.
The ISU rules define the take-off edges: the flip takes off from a backward inside edge, the Lutz from a backward outside edge [4]. A feel for backward edges is therefore the basis for these jumps.
Bend the skating leg, keep weight on the balls of the feet and practise first while standing and at the boards. Fall sideways, never onto the tailbone [1].
The Two-Foot Spin: Your First Spin on the Ice