Well, I'm no expert, but in '03 - exactly 10-days before closing on the sale of my house(and the purchase of this one) - I discovered I needed a new septic system that was quoted at ~$10k, which coincided with all the profit from the sale(which was to be the downpayment on this place!).
I did what any GRMer would do, and designed and installed it myself, with the help of a few friends. We ended up pushing closing out a couple days, but it ended well & my total cost ended up being $2500.
I did have to meet code & pass an inspection, though the inspector was very easy to deal with(I had a feeling they have people doing this on their own very often). Overall, it was hard work getting it done in time, and making it work in our yard, but it wasn't difficult to design the system.
Here's what I can remember:
1.)The size of concrete tank, and your runs, is determined by the number of bedrooms. I'd guess your county health department would have the specs, even if you don't need to follow them.
2.)E36 M3 rolls down hill. Seriously, remember that & you'll be OK. IIRC, I only needed a 2" drop in 12' or maybe even less.
3.)I bought the tank & all the pieces for the runs(I don't think they call them leach-fields anymore, but they basically look like small, perforated, ABS plastic dog houses about 1' tall x 2' long that interconnect) from the same source. They delivered it all & placed the tank with their crane.
4.)I rented a mini-backhoe for a couple days & had a friend who operates heavy equipment come run it for a day & teach me how to use it. It barely reached to the bottom where we needed to put the tank, but we did get it done.
5.)I borrowed some survey equipment from another friend who's a geologist, who also taught me how to use it. To me, one of the hardest parts was getting the slope correct & even for the runs, yet also not going too deep(IIRC, we couldn't be more than 36" under the surface, which was a problem since our yard sloped uphill).
I'll dig around and see if I can find my envelope full of notes & plans. Not sure if there's anything else in there that may relate or be helpful, but I'll check. I also remember finding some online reference & help at the time, not sure if I have those URLs in there or not.
Good luck!