Project tutorials -  these are the tuts that yield an asset when you're done - from asteroids to medieval towns.

Beginner

 How to Create a Low Poly Tree in 1 Minute -- Uses some of the ideas in the above link.  Watch the above link first Then make a tree using this method. 

Making grass fields.  Using weight maps and modeling.

Make an Ocean --

Best Ocean yet - his ocean shader looks promising

Intermediate

He uses the particle system to make a simple low poly low image leaf texture attach to branches.  Learn how to assign vertex groups (shows where to attach the leafs) 

Here is making sea foam in an ocean, great for boat wakes.

Make a mountain using micro Displacement.  He uses the same technique to make a Moon with mountains and craters.

Create a realistic asteroid - this one yielded a decent result.

Create a realistic asteroid  this one makes an asteroid with impact craters -  needs to be less spherical though.   Maybe this asteroid needs more of the detail from the first one combined with the craters from this one to achieve something more realistic.

Create a realistic asteroid - well it just looks like a rock actually - to make it look more like an asteroid it would need to look more smooth with dust and have some impact areas - he doesn’t really do that here but it's enough to get that rock look - i’d combine this tut with the tuts that make impact craters, also I’d do small rock instancing on the hero asteroids as well.

Expert

medieval kitchen - some insights - not great for actually step by step though -

1 hour create a medieval village - there are quite a bit of these - however its for 2.79 blender - make note of the version because blender changes drastically from version to version - trying to hunt down the tools in 2.9 from 2.79 is a real problem.