The cows and ponies of the Highlands were quite small and their sheep certainly continued that pattern. Their modern descendants, the Soay and Shetland breeds, are only knee-high. These relatively primitive breeds can shed their wool but are most often sheared like other sheep. After shearing, the first step of preparing the raw fleece was (and is!) to "skirt" or remove the pieces with excessive hay, burrs, or poop. The long fibers are coated in lanolin, a waxy oil, which is usually washed off in modern preparations but was kept for its waterproofing. After a cold soak to remove urine and feces, the fleece is dried and then combed or carded. Combing or carding aligns the fibers for easier spinning. Once prepared this far, the wool is ready to be spun on a spindle.
Spindles of this era were not much more than simple sticks. In the late medieval period, a medieval bellied spindle with removable weights would still be used along with a long distaff to hold the fiber. This was succeeded by a more modern drop spindle - a straight shaft with a fixed weight. The dealgan was a uniquely Scottish spindle design where the bottom of the spindle widened into a square which had a cross etched into it. The dealgan was likely used solely to ply yarns together rather than to form a thread from loose fiber; they're simply too heavy to spin thread. Indeed, thread rather than yarn was what they needed to spin - that thread would then be sent to the guild weaver to by dyed and woven into cloth. That cloth would be all the family could use to clothe themselves. Each garment was an investment of months of labor.
Why not use a spinning wheel? Wheels took centuries to come to the Highlands even after becoming the primary spinning tool of the lowlands. Several compelling reasons kept the spindle in use even into the 19th century: a lack of large pieces of wood on the deforested island, complexity of construction and repair, and portability. A spindle could be (and was) carried anywhere and used any time the hands were not otherwise engaged. It was simple to build, as it was little more than a single short stick, and therefore easy to replace. By comparison spinning wheels were complex, large, and required single-minded focus on spinning for an extended period. That said, the muckle or walking wheel did arrive in the Highlands in the 1700s. The walking wheel was so called because the wheel had to be turned by hand; the spinster would start with one hand on the wheel and the other drawing fiber back from the pointed metal spindle, and she would walk backwards while turning the wheel and drafting thread into being. When she was as far back as possible, she would reverse the direction of spin and walk forwards to wind the new thread onto the spindle. A spinster was said to walk as much as five miles a day in this manner.
The other fiber used in Highland clothing was linen grown from the flax plant. This process was at least as arduous as that for wool.