When I first started, I had a pair of dividers, an overstitch wheel, a scratch awl to punch the holes, craft needles and some nylon blind cord. I worked at a factory that made blinds and dug rolls of rejected blind cord out of the trash.
Not a very good picture but one of my early attempts.
I saved up and got more tools as I could afford them. Now I use either a stitch groover or the dividers, stitching chisels with 3mm spacing and 000 needles. I have a variety of waxed threads to use depending on the look I want. I use a stitching awl if the leather I want to stitch is thicker than my chisels are long.
I still don't have a proper stitching pony yet. Instead I use a wood clamp on my work bench.
Eventually, I had so much stuff I had to make a box to hold it all.