Start Here
Most of the model horse hobby seems to be populated by do-ers and DIY-ers. Something in our wiring - the part that's connected to the love of little things - wants to MAKE stuff. We can only spend so much time looking at painted models, customizing blogs, and gorgeous tack before we want to try it ourselves.
Making a tiny saddle is a daunting prospect, even for hobbyists who are good at other crafty things or the other creative aspects of the hobby. It's pretty daunting even when you have a solid working knowledge of the real deal and how it works. I have a pretty good stash of resources piled up, so I want to share them with anyone who is thinking "I want to try, but where do I start?"
Start here.
Complete Kits
There are a handful of kits that include everything you need except the tools and glue. That's a great start for anyone who is coming at this from the ground up. I've linked the two I know of here:
Rio Rondo (western saddles, 1:9 & 1:12th scale)
The Unicorn Woman (Melody Snow) (english saddles, 1:9th & 1:12th scale)
Instructional Materials
These books & PDFs do have a cost, but the information in them is usually worth it! They don't include any of the supplies or hardware required to make the tack, but they DO include patterns and instructions and are, as far as I know, completely DIY (including the tree!). Susan Bensema-Young's book should be required reading for all tack makers; there's a wealth of information in this book and it's clearly written and easily followed.
Timaru Star ii Guide to Making Model Horse Tack (all disciplines, 1:9th scale)
Dreamflite Design English Saddle Guide (english, 1:32nd scale)
Keri-Okie Entertainment (all disciplines, 1:9th scale with directions for modifying for others)
Free Online Resources
Hobbyists deserve fair compensation for the work it took to develop their pattern, instructions, and the time involved in producing a finished kit or guide, but sometimes when we're just starting out we simply don't have the funds to support them. There ARE free resources online, but you're not likely to find a finished pattern and instructions. Occasionally, you'll find a complete walk-through but more often than not what you get are wonderful tidbits of information that can really help streamline your process or get you through a particularly tricky thing you've been trying to puzzle out on your own. In most cases, these bloggers & websites have tagged their material in a way that makes it easy to find the stuff directly related to tack making; where possible, I've linked to those areas.
Saddle Tutorials
Saddles with Sophie, Part 1 & Part 2 (an english saddle from start to finish; no pattern)
General information
Braymere Custom Saddlery Blog (Jennifer Buxton - primarily 1:9th)
Brazos Ranch Studio (1:9th)
Desktop Stables (Nichelle Jones, 1:9th)
Shoestring Stables (Leah Koerper - primarily 1:32nd)
Stage Left Studios (Grace Ledoux - primarily 1:32nd)
TiME Stables & Studio
The Unicorn Woman - Free Articles (Melody Snow)
Print-Only Resources
Most of these have been discontinued but can still be picked up second-hand through various hobby channels.
KSmith Tack Makers Journal (print magazine that included CDs; now available directly from Kim as a 255pg PDF)
Model Horse Performance Magazine