I recently bought a kit for towels. The thread is wound on spools. They don’t fit on my cone holder, so I’m wondering what is the best way to wind the warp using them?
You wouldn’t want to place a spool on a cone holder anyway. The cone holder would hold the spool upright and the thread would wind off the end of the spool, putting a twist (or an untwist) on the thread as it comes off the spool (read more on that phenomenon here). It’s better for the spool to rotate as the thread unwinds. Spool racks, with rows of rods, are designed for this purpose. If you don’t have a spool rack, you can rig up a temporary one (that can also double as a cone holder).
For this, I use a plastic file crate, available from office supply stores. Turn it upside down, and slide a rod (an untwisted coat hanger would work if you don’t have anything else) through the slats, placing the cones on the rod. The yarn can then be brought up through one of the holes in the crate to unwind directly above the spool. Cones can also be placed under the box and the yarn fed through a hole above the cone so that it unwinds directly from the cone (or from more than one cone at the same time without tangling).