There are some hand shampooing threads. I am a small op, but I work hard.
1. Hot water + Automagic HD Upholstery Shampoo according to directions
2. Vac mats, soak, let sit for 4-5 mins, agitate hard with a brush, pressure wash, extract with a shop vac (a 6hp would be lots I think, even 5hp would be ok, but frustrating)--let the mats dry
3. Vac car
4. Pretreat stains with stain cleaner (OTC), an APC, or enzyme, depending on the stain and colour of carpet
5. Spray the shampoo on the carpet, agitate by hand, extract with shop vac.
That's what I do. I've rented a machine, and found I did just as good, so the next step for me would be to get a $1000 machine designed for cars. I undercharge, underpromise and do the best I can.