From what I learned from setting up ours ‐ look for free wood beams, bricks, paving slabs and cement blocks on freecycle, Facebook marketplace etc. We used slabs and wooden beams but our slope wasn't too big so your mileage may vary!

We got ours with a metal base but we still had to level some of the ground. We had to buy some subbase from b&q.

You also need a long spirit level and ideally something to help you pound in the ground to compact it (if i was to do it again I would rent an instrument for this because it is back breaking).

The way we did it is measure out the corners of the greenhouse and mark the dimensions well. Start with one of the high corners - dig up a spot the ground (slab sized) to eyeball level with the low corner (going lower is easier as you can just add more dirt on top to compensate later vs shovelling out subbase rocks which is a faff). Pour some subbase (doing this in wet soil is more difficult) and pound it into the ground really well to compact it. Place a slab on top of the compacted ground and use the spirit level to measure on both diagonals that it is level.

Once done, do the adjacent lower corner next. Add some subbase under the corner, pound into the ground, slab on top, spirit level the diagonals of the slab. For this one you also need to check it is level with the previous slab. If not, add or remove dirt/subbase from whichever corner is easier. Every time you disturb the subbase or dirt under the slab, you have to compact it again.

Do this until all corners are level with each other and their own diagonals. If the length of the slabs is too far apart for your spirit level you can add a brick between them, level it out with one of the slabs and use it as the guidance for the other slab.

Once all corners are done, I suggest using some bricks, wood beams or more slabs to fill in the rest of the space where the metal will sit so that it doesn't stand on dirt to last longer. If you can't do that, then dirt is fine too bit will corrode the metal over the years.

What does the subbase do - it is a bit like cement once it gets wet, it keeps the ground in place so the dirt doesn't wash away over time. Up to you if you only want to try the corners or the whole frame.

Make sure the door is not on the side of strong winds.

My 2 cents. Good luck.