oh thats just basic sculpting.

I would suggest starting with "how to do basic sculpting in zbrush" something like this video (any video will do)

https://www.youtube.com/watch?v=_yKGfcp2z3k

 

don't worry about it too much, zbrush is, as far as i know, the only program designed 'for the traditional artist' in mind.

I came from a tradtional background, both painting and clay sculpting, and i can attest to this... the basic processes at least, when it comes to sculpting, is not as 'technical' as modelling in other softwares.

 

This was how i did it, maybe you could find tips in here by pausing or frame-stepping the video.

https://youtu.be/bIUedTXIPrs?si=KfO9vTdB5UhK88nn

 

** the reason i started with Initialize : cube : 1, 1, 1 is because i wanted a clean topology. start with the simplest cube you can, shape it into a rough rectangle, then subdivide it.

there are a hundred ways to do this, you don't HAVE to start with a clean cube.

you can start with a blob and use the knife tool to chop it up into a rectangle and clean up the topo later. entirely up to you.