Replacing Sleepers with Rock Edging

Hi Guys

We recently bought a new house and currently have railway sleepers for our garden bed edging. We want to change this to rock edging (I have been told that sleepers can potentially attract termites BTW!). I plan on getting the rocks delivered & will then build the rock edging myself. The rocks in question will be various sizes & standard 'edging' / 'volcanic' type stone which I have seen used elsewhere.

If I'm replacing the sleepers with rocks do I need to do any of the following?

1. Dig a trench to place the rocks in (or can I just place these on the ground where the sleepers used to be?)
2. Mortar / cement the rocks together to form a barrier / mini-wall (like a sleeper) or can the rocks just be laid loose, then filled in with smaller rocks etc?
3. Any other advice would be very much appreciated, thanks.

Cheers...