This document describes how to easily get uniform, precision lines from your plastic mehndi cone by using readily-available metal tips. This probably works better with "carrot bag" style cones, both those made commercially as well as homemade ones.
Most mechanical pencils sold today, especially cheap ones, have metal tips in a plastic housing (where the "lead" comes out). Typically, this tip can easily be unscrewed from the pencil. These tips have either a .5mm or a .7mm opening, and are ideally suited for fine lines as needed in mehndi work.
The primary benefit of using this technique is the ease in achieving a fixed line size without having to guess at trimming the tip off of a carrot bag style cone. Likewise, the opening won't stretch during extended usage as is sometimes seen when simply trimming the tip off of a cone. Another benefit is that mechanical pencils come with a small wire which fits the tip exactly...this makes it easier to clear the inevitable clogs (I've found that poking a wire in a metal tip much easier than poking a pin in the tip of a plastic cone, but maybe that's just because I drink too much coffee *grin*).
First, go to any store which sells pens and pencils. Review the selection of mechanical pencils, picking one that is of suitable size (they come in .5mm and .7mm sizes). Try to verify that the tip will screw off (this may be difficult to positively verify, but most do). Make your purchase(s), as theft debits your karma more than the cost of a mechanical pencil.
Disassemble the pencil, finding the little metal wire for cleaning (typically under the eraser), and unscrewing the tip housing. Use the wire to remove the lead from the tip. The picture below shows all the relevant parts (the tip, the cleaning wire, and the plastic cone).
Snip off the tip of the cone, making an opening large enough for the tip housing to slide about halfway out before getting snug. Then, using the leftover pencil body (or a pen, or another pencil), push the tip tightly in the opening. The plastic will stretch around the tip housing, creating a snug, friction fit. Be careful not to stretch the plastic too much in the area immediately above the tip housing.
Fini! Fill 'er up with paste and enjoy!