The pistil stitch is really easy! It's a lot like French knot. In case you don't know how to do French knot, here is a tutorial for both that and pistil stitch.
Pull your needle and thread through the fabric, then wind the thread around the needle 2-3 times.
Hold the thread firmly with one hand so it is tightly wound around the needle, then insert the needle into the fabric again and slide the needle through the 'knot'. Hold the thread taut so the 'knot' doesn't loosen - but don't hold it so tight that you can't move the needle through it.
Not sure where to insert the needle? My rule of thumb is the width of the thread I am working on. So a thin thread means the exit and entry points are quite close together, and with a thicker thread they are further apart.
Hold the thread taut (on the left) as you pull it through the knot (on the right.
I just use a regular embroidery needle when I do French knots, but some find it easier to use milliner needles, so give that a go and see if it makes a difference.
If I have trouble getting my French knots right, it's usually because my tension is wrong. If that thread isn't held taut, the knot starts to misbehave and things go messy.
Instead of inserting the needle right next to the exit point, insert the needle at a distance to make a nice 'tail' on the stitch. Anything up to 1" /2.5cm from the exit point should work. Anything longer than that and the tail will start to look kinda floppy.
Step 3: same as for French knot, see above.
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