Hollow block raised bed

Can anyone advise on how to lay a double row of hollow blocks to create a raised bed.

We have a raised bed at the moment with a cherry tree in it, which we created using sleepers a few years ago. Unfortunately the sleepers haven't really stood the test of time and have been slowly rotting from the inside and have started to crumble (see pic).

We are having some hollow blocks delivered in the morning to create the new bed. We plan on it being 2 blocks high. It only needs 2 sides doing as it's pushed into a corner of the garden. It will be 3 blocks wide along one side, and 4 on the other. Any advice on how to lay them? Will they need mortar?

Many thanks

I recently laid a raised bed using solid, rather than hollow blocks as these are less deep, leaving more space for the soil.
This is two blocks high but, knowing that mortar is meant to keep blocks apart, I laid the top course on to the lower course using Gorilla Glue instead, as I thought this would hold them together better (I expect some criticism for this!). I did, however, use mortar to fill the spaces between adjacent blocks.
There are no 'foundations' as the ground is quite solid clay.
Before doing this, I had to dig out a large and well established Pampas Grass plant. That was hard work.

Thank you JBR. I've only just seen your reply. Nice job. We laid the first row yesterday using mix in the tub mortar, and today the second layer using donated postcrete (it was free and beggars can't be choosers!) I actually sat there sieving out all the gravelly bits first!

Just need to point in between the blocks, then paint it with masonry plaint.

Now if anyone can recommend some shade loving trailing plants we can plant in the holes that would be great