A little goes a long way.

Stainless steel fibers blended with wool roving and spun and plied on on a drop-spindle.

Photos: https://www.flickr.com/photos/plusea/albums/72157711023086991

This is not a recipe but an attempt to share things to think about when you approach spinning your own conductive yarn. Or “sensor” yarn in the sense that it’s electrical conductivity (or resistance) depends on how well the short steel fibers touch each other: make physical contact for electricity to travel through.
Changes in resistance are due to changes in these contacts: the surface area (due to pressure) that is touching each other.

In this post on Speculating about Piezoresistance I attempt to describe what in even more detail what is going on “inside the yarn”.

REFERENCES

Irene’s Posch’s spun conductive yarns:
>> https://www.stitchingworlds.net/experimentation/spinning-electronic-yarns/

TOOLS

Troy’s DIY spinning wheel:
>> http://www.fabacademy.org/archives/2015/eu/students/nachtigall.troy_robert/Project.html
>> http://www.fabacademy.org/archives/2015/eu/students/nachtigall.troy_robert/Final.html

Cheap toy?
>> https://www.dreamingrobots.com/product/eew-nano/