A 20mm slate aggregate is excellent for paths as it is good for forming a flat and comfortable walking surface.
The proposed path area needs to be dug off to a minimum depth of 100mm and ensure the removal of all weeds and other unwanted organic matter. If the area of the path is troubled with weeds, the excavated sub-grade may be treated with a general weedkiller. An MOT type 1 hardcore or weed membrane [ this is optional] will also help with future weed suppression.
Edging to the gravel area is necessary to keep the gravel you lay compact and prevent its migration to other areas. This can be constructed with a treated timber edge combined with peg supports or alternatively using hard landscape edging in stone or concrete.
The base layer to the path provides the strength and competence of the structure and should consist of a 75-100mm layer of crushed MOT Type 1 stone, that is ideally compacted using a vibrating plate compactor.
[ Note Typically 1 tonne of DTP1 granular sub-base covers approx 6-8 metres square at 75mm compacted thickness.].
Slate provides an angular gravel and will give a greater degree of interlock, and so will be more stable underfoot than many other decorative aggregate types. Once the subbase is complete the 20mm angular slate chippings can be applied directly, and then raked out to the desired level. The top dressing will typically be 40-50mm thick. You can if necessary, then compact the top surface again with the vibrator plate to ensure it gives a firmer surface, and then top up as necessary. [Coverage rates for slate are typically 20 metres square per tonne at 40mm thick.
Once complete it is a good idea to thoroughly hose the slate that has been laid as it will help to remove any dust or fines on the surface to the bottom of the applied layer.