There are two belt paths for installation, short belt path for over 40lbs, and long belt path for under 40lbs. You can use the short belt path for under 40lbs too, and when my boys started using the Regent they were around 35lbs, I just installed short belt path to make sure they didn't pass up 40lbs with the long belt path installation. At this point I would skip the LATCH installation because most cars LATCH only works to 40lbs, and he's close enough to that to not bother.
First put the recline bar on. It won't stay in on its own, so get a rubber mallet and pound it in a little (it will say this in the instructions too.) Then sit your son in the seat and adjust the harness to be at the slot that is just at or just above his shoulders. You do not want it below his shoulders for forward facing, this is dangerous and in a crash if the harness is below his shoulders can cause spinal compression.
once you get it adjusted to fit your son in the house, take it out to the car for installation. If you run it the long belt path, the seat belt goes up over the seat, then down at the bottom through the belt path, then back up over the seat and buckle the belt. If you choose short belt path, thread it through the back of the seat and through the seat (make sure it goes under the padding and cover, don't go above the cover or padding for short belt path, the belt should be against the plastic). Then back the seat and buckle.
Read your vehicle manual to learn how to lock your seatbelts. Once you get the belt threaded through and buckled, lock the seat belt, then pull on the shoulder portion of the belt by the buckle to tighten the lap belt. Push the front of the seat in with your knees so the recline bar goes into the back of the seat, then climb into the seat and use your body weight to tighten the belt down, and then let it retract back into the car.
THe Manual can be confusing, but it does have some good pictures. Where are you located? There are alot of SAFEKIDS checkpoints in the MPLS area, a good one in Rogers at the Chevy dealership that is a Safekids checkpoint. If you were closer to me (I'm near Monticello) I would help you! I can get the Regent out of one car and into antoher in less than 5mins.3yrs of practice doing it!
Good luck, and if you need hlep find a safekids contact, read the manual step by step while installing, and send me a pm if you have a question.
And don't forget to hook the top tether! Its very important and required over 40lbs!