Welcome to permies Kate,
The obvious way to build this land into good soil would be laying down topsoil, expensive and not really necessary unless you are working with time constraints.
I would use the method of composting/mulching/planting to build topsoil within the boundaries of what is already there.
Depending on the quantities of compostable materials you have or can get free, you may need to take a cellular approach where you do small areas at a time.
You could bury
wood, and keep the surface nearly level, or you could build some actual hugels around, either will work. The dependent is your intended use for the land.
Pastures do not have to be level, they can have bumps and wriggles, which give places for
water to soak deeper into the soil/substructure.
Layers work well for building soils,
lasagna mulches end up making
compost which helps build new soil. You can also plant into these lasagna mulches giving new green material which can be chopped and dropped to continue the process.
This website is very good as is richsoil.com a web search for
permaculture will bring up several other sites to peruse.
One of the best methods for building soil into humus rich, healthy soil is to lasagna bed it and plant deep
root plants into the applied composting mulch beds. When these plants have grown just chop them down and leave the below ground
roots to rot.
These decomposing roots will add a lot of organic material into the soil, help it both drain and retain water and also give places for the necessary micro organisms to grow and thrive.
Even untreated cardboard and black soy ink newspaper are good for amending soils, they keep weeds down by blocking sunlight and they decompose creating organic materials to be adsorbed into the soil they covered.
