This is an extreme close up of the blocks I am working on. I use invisible thread. I use any of the extra fine ones on little cones, I find the ones on the spools are too tightly spun onto the spools and come off the spool under tension. This makes them spring out of the route down to the needle on my machine. Some machines are less sensitive. I use a heavy cotton thread in the bobbin and a Size 60/8 Schmetz Microtex needle. You can get away with a 70//10 if your bobbin thread is thick enough. The idea is for the hole made by the needle to be so small that none of the bobbin thread is pulled through to the top of the work. For the same reason I loosen the top tension as much as I can and tighten the bobbin tension as well. Contrary to what you would expect this very fine thread does not slide easily through the tension disks so the top tension normally needs to be lowered quite a long way down whatever stitch you are using.
This shape has been prepared with glue folding over the edges like I demonstrated in an earlier post. You can see where the snips into the convex curves have left that edge in those places quite vulnerable.
I am showing the method using a coloured cotton thread so you can see it clearly. You need to use an open toed foot so you can fully see where you stitch. Take your work under the foot and bring the needle down and up just where you wish to start stitching.
Hold onto the top thread and gently pull the bobbin thread through the work. Drop your foot. You are now ready to start stitching but you need to hold onto those threads for the first few stitches. Lower your foot and do 3 or 4 stitches in the same place by reducing your stitch length down to zero. Then put it back to the length you want to work with.
The stitch I use is the blanket or buttonhole stitch. That is one stitch forward, one to the left, one stitch forward and so on. You might not have an identical stitch on your machine but you probably have something similar. At the very least, all machines made in the last 30 years or so have a blind hem stitch, which is 3 or 4 stitches forward, one to the left, then 3 or 4 stitches forward and so on. One my machine for the blanket/buttonhole stitch I have the stitch width just below 1 and the length just below two. If you are using the blind hem stitch you will have to reduce your stitch down very very low (the downside of this stitch is the very tiny stitches are a little more visible than one larger one). So that is the setting MOST of the time.
Because I am stitching in cotton so you can see the stitch I have doubled the width in these samples. It takes a little bit of practice to get down to a very low width, so start with a wider one and reduce it little by little as you get more confident.
Turn the fabric gently as you go round curves but that will not get you right the way round and edge like this. If you have the 'needle down' option on your machine, now is the time to use it. If you don't have the 'needle down' option, try tapping the bit of the foot controller closest to you. Some machines enable you to 'tap' the needle down like this. If neither is an option, then use your left hand to hold the fabric in place whilst you turn the needle down with your right hand on the wheel. Then lift your foot and turn the fabric a little. Lower it, do a few more stitches, steering with your left hand, then needle down, lift the foot again and turn the fabric. You need to do this often. This isn't fast stitching. Faster than by hand but not a lot.
This is my first and middle finger helping with the steering.
When you do the concave bits, increase the width and shorten the length of stitching just for the area where you have snipped the fabric on the other side.
The tighter the curve, the more you have to stop and lift the foot to turn the fabric. On very very sharp bends it might even mean doing it every stitch until you are past the tight curve.
From the back you can see when I move from one area of stitching to the next, I don't cut the threads unless the distance is more than 4" or 5". When I get to the new stitching, I still do the few stitches in the same place to hold the start and then when I have stitched away from the start I cut the top thread very close to the top with curved nail scissors flat against the surface.
Practice on some heart shapes whilst you get the 'feel' for the stitch.