The most common time that you want to put piping into a seam is usually around the edge of a pillow and there are lots of good tutorials on You Tube for this. But for the embroiderer sometimes the scale is a whole lot smaller, as in pin cushions. The diameter of this one is only 3". (There is no way I would go any smaller than this.)

You could just cross over the ends of the piping but it you want an invisible join it isn't that easy and is certainly not for the faint hearted. In fact I probably should have put piping at the bottom join as well to make this look balanced but as this is only a demonstration I chickened out and only did the top edge. It is the small size of this piece that raises the level of difficulty. So how do you do it?
I start by selecting a couple of contrasting fabrics. Once for the main and a contrast for the piping. I allow extra for the seam allowance, usually 1/4" and cut 2 circles for the top and bottom and a strip about 3" wide for the sides. Once you put the top and sides together you can adjust the height.

Select a width of cord that is in proportion to the piece you are making. My cord is fine. I laid this around the line I drew on the back of the top and bottom circles. I make it a little bigger to give myself some margin for error. (I do this by laying it on the outside of my line.)

I cut my contrast fabric on the bias 1" wide and attach one end in the middle of the bias binding. This is just to hold it in place. (I will remove these stitches later.)

Fold the bias strip in half to enclose the cord and stitch about 1/4" from the edge. Again, this just holds the cord in place, you are not stitching up close at this stage.

Mark the centre of your cord with a pin.

Pin the covered cord around the edge of the top circle, pining either side and working out from the centre mark, on the right side of your fabric Inside the line of machining and snip your edges to allow the piping to stretch around the circle.

. Check that you are matching this to the line your have drawn on the back.

Continue until you are able to join the covering fabric. You will need to keep the ends of the cord out of the way and join with a bias seam so that the fabric will measure exactly. Take time to do this. Rushing will just mean unpicking.

The next step is to join the cord. On a pillow you could just butt the ends together and sew around your piping but on this you have to butt the ends together and then darn the join. If you join it with a seam it is just too bulky and if you join it in the usual manner the ends come apart because the curve is so tight . Nothing else for it than the darning.

Sew the binding edges together and pin in place.

Now tack that covered piping to the top circle. Compare the pinned image above to the image below. You can see how much better that piping sits. This will make sewing the piping in position far easier and it will be a lot more accurate.

Sew the piping in position on your tacking line. That is inside your line of machining. This will make your piping sit better in the casing and give you a reference line when you come to join the side. Trim up your edges so that they are all neat. Remove your tacking.
Join the seam in the side piece to make a tube and pin in position.

If your are new to this I would suggest tacking the edges of your top circle with the binding attached and the sides as well. These edges are known to move. Sew around the edge using the stitching line that is there as you guide.

I normally stitch one needle width inside this.Your piece will look something like this. Now is the time your can adjust the height to suit.

Sew the bottom circle to the sides. (This is a lot easier without the piping.) Nick the edges to allow the seam to sit better.

Open your side seam enough to allow you to pull the fabric through to the right side. Stuff if fibrefill or what ever filling you select. Close the opening with a ladder stitch. (Gee I'm glad that is finished.)

This is a lot easier if your piece is larger. Darning the piping cord ends together before your cover the cord will give you a better finish that isn't bulky and you will not see the join or the raw ends.