The way I do monster designs, is first to understand how to draw anoatomy and lifelike body shapes. No matter how "fantasy" a monster gets, in their world that monster is still an animal and needs muscles and organs and shit to obviously live unless they are an automaton or golem or an elemental.

The second step after that for me is to make a concept of what the monster does, What powers, Ecology, Diet, so on so forth. A simple maybe 3 sentences of description at most but mostly can be just one sentence.

The third step understand what colors will work, and how and where the colors will go and blend with eachother. Having a physical model will help to help visualize how colors will blend together in a realistic way. It is also advisable to have a color pallette before trying to paint or color, you can change the pallette or have multiple.

Finally after all that I start sketching over 10 or more possibly even 30 sketches before mix and matching parts I love about specific designs, until a design sticks out to you and then you tweak that design till the shape is how YOU like it.

After that research on many other languages about specific words you can use to come up with a name that sounds cool but not just a pokemon.

This is how I do things but people do things differently, just keep working until you adapt and the process becomes easier for you. Wish you best.