The slide position decides your balance: sliding foot under the sternum, trailing leg extended, broom as support. With checks, evidence and milestones.
The honest answer: a good slide position is mostly a question of balance, not strength. Curling Canada describes the slide like this: it begins when you leave the hack and ends technically when the release motion begins. The key to any successful slide is good balance, that is the dynamic stability of stone and body as they move towards the target [1].
That balance is trainable and measurable is shown by a Swedish study with 20 super-league players: a newly developed test in the delivery position (deep lunge, front foot on a tilting plate) clearly differentiated elite from sub-elite players [2]. The authors consider balance in curling a separate, task-specific skill that must not be confused with single-leg standing [2]. Note: the two groups also differed in age, playing experience and training frequency [2].
How hard is it? You should already be able to glide (see the sliding guide). This is about the clean position within it. There is no timeline.
When planting, the sliding foot sits exactly on the target line, the inside of the ankle aligned with the middle of the stone [1]. It sits under the vertical body midline: under the sternum for a flat slide, under the navel for a raised heel, under the belt buckle for toe-tuck [1]. The beginner handout puts it as: sliding foot under the centre of the chest [3].
Milestone: In the side video the sliding foot is below the sternum, not out in front.
The hips stay square, the trailing leg is fully extended and lies entirely within the outer edges of the stone [1]. The anchor foot is as square to the ankle as possible, ideally with the tops of the toes on the ice [1].
The beginner handout describes a tripod support of broom (45°), sliding foot and trailing toe, which you can even practise on the living-room carpet [3]. The broom stays in the same position as in the set-up and the drive [1].
The shoulders stay square to the target line and level with the ice [1]. The head sits centred on the target line; one eye can be over the middle of the stone [1]. The throwing arm stays extended in the first part of the slide (roughly between back line and tee line) and softens afterwards (between tee line and the top of the house) [1].
Slide without a stone, hand off the ice [3]. Check: do you lean on the stone, wobble, reach for the ice or slide onto the hack knee? According to the handout, these are the signs of lacking balance, and they usually mean the sliding foot isn't under the centre of your body [3].
Milestone: Five slides without a stone and no hand contact, then five with a stone.
In a study of five elite and five sub-elite curlers, elite curlers showed larger ankle dorsiflexion (toes pulling towards the shin) and a larger contact area of the sliding foot [7]. Sliding time and the speed of the body's centre of mass did not differ there, but sliding distance and speed profile did [7]. So your lever isn't "faster" but "plant broader, keep the ankle mobile". It is a small sample.
Exactly on the target line, the inside of the ankle level with the middle of the stone [1]. It sits under the vertical body midline: under the sternum for a flat slide, under the navel for a raised heel, under the belt buckle for toe-tuck [1].
According to a pilot study, knee joint force when sliding on the ball of the foot is more than double that of flat-foot sliding [4]. In professionals, toe contact also increased hip and knee torques [5]. For beginners, flat is the more cautious choice.
Slide without a stone with your hand off the ice [3], and use a phone video to check that the foot stays under the sternum. One study considers curling-specific balance a separate skill that differs from single-leg standing [2]. So practise in the delivery position.
According to the handout, that is a sign of lacking balance and usually means the sliding foot isn't under the centre of your body [3]. Position it under the chest and practise without a stone [3].
Holding the Line in Curling: Body and Stone on the Broom