Hi @May. Many thanks for your question, I'd be happy to assist with it.
I would advise compacting the soil and then doing the same to a 50mm layer of gravel on top. You can then add sand over the gravel to assist in levelling the sleepers. If you lay the sleepers directly on the soil there is a chance that they will sink unevenly overtime creating a trip hazard. The preparation of the ground is to ensure a solid and sturdy foundation for the pathway to be placed on.
If you would like further information or had other questions, please let me know.