Designing new characters is hard! I can't do it in one step. So I draw a character, and make successive new versions until it sucks a little less. Below you can see the document I'm working on, with tileset and background from the current version of the game, which allows me - hopefully - to draw new things in the same ambiance, atmosphere or color scope (this is hard but extremely important).
3 tries of new monsters. A rough draw of a golem, a blob and a spider. Blob and spider are in their third or fourth version, but not finished yet.
Let's focus on the golem. Below is the second version where I add a light-emitting heart and redraw the sprite (the previous version is used only as a shape) with 3 brown colors.
Below is the next version, adding one color and refining shadows and lights.
Here is where it's useful to work on a document with other elements from the game for reference : I change each color (mostly about saturation) to have a character better fitted in his world.
The result is below :
Next thing to do : animate this pile of rock.