There are in fact a couple of ways to keep your stitches even:
1) The stitch marking tool that uses a small wheel at various sizes such as 6 stitches per inch (probably the most common size used today). A variant is the overstitch wheel which can be used both as a stitch marker but also used after sewing to smooth out the stitches - thus the name over stitch wheel. I recommend getting a decent quality such as those by CS Osborne - unfortunately the ones currently available from Tandy/LF leave a lot to be desired and those ones with three interchangeable wheels are frankly crapola....
2) A pricking iron - this is a punch type tool that lays out not only your spacing, but also the angle for the awl to be used at. Again it comes in various stitches per inch.
re: stitches per inch - if you're doing "period correct" leather work to the nth degree then you will be using a finer thread (no more than 3 cord linen or hemp) and a stitches per inch of 8-12 per inch rather than the "coarser" 5-6 commonly used today with thread in the 5-7 cord range.
here's a sewing tutorial I wrote sometime back that may give you some hints - it's a variation on the traditional harness/saddle stitch - some things are dated such as wher to buy, but the rest is OK
http://www.wrtcleather.com/1-ckd/tutori ... titch.htmlas for thonging chisels - with all due respect they do work, but can (and will - FWIW that's based on over 50 years of leather craft and repair) cause problems over time since the holes are set up differently than for hand sewing - the main problem being over time that the stitches will tear through since the "holes" are lined up directly between the stitches - kind of hard to explain in words but hopefully ya'll will understand
Where I get my tools/supplies - mostly from Campbell-Bosworth in Texas. It's one of the few places offering linen thread (the Barbours) and the tools are good mid-range CS Osborne tools. There are also custom tool makers such as Douglas which are excellent - the only down side being the price. Another option is vintage leather tools which can be found on Ebay, Proleptics, Brad Johnson, etc. While vintage tools can be excellent and often at a good price, you do need to know what you're looking for.