Now, as to how I approach things:
1) Come up with concept.
2a) Look at the stats of any critters of roughly the same size and power as my concept.
2b) Compare them, and use figures in the same ballpark.
3a) Look at critters with the same or similar special abilities/attacks.
3b) Use figures/rules in the same ballpark.
Here, let me talk you through an example from a previous thread:
Someone asked how you'd stat up the massive giant on page 45. Here's what I did...
1) Looked at the pic, judged its size based on the humans standing nearby. I decided his head was roughly 5'-6' long, so that'd make him 40'-45' tall.
2) Looked at the giant entries in the DCC rulebook. Turns out, our pictured giant is twice as big as any giant listed there!
3) Looked at how the size of the giants increased from type-to-type and how that adjusted their HD, damage, and all other stats.
4) Looked at the stats for the other massive creature in the book, Dragons (had a handy chart there too!).
5) Created stats based on those comparisons and examples:Giant, god-born
(42’ tall, 50,000 lbs.): Init +3; Atk sword +32 melee (8d8+16); AC 23; HD 26d10; MV 70’; Act 6d24; SP single immunity and three powers based on divine ancestry (one usable 3/day, one usable 2/day, and one usable 1/day), crit on 20-24; SV Fort +20, Ref +8, Will +14; AL as per divine parent.
I can go into a more detailed, blow-by-blow account if folks want but really, it comes down to 1) conceptualize, 2) look at existing creatures in the same type/ballpark, 3) use those as guidelines and modify. I seriously recommend folks give it a shot. It can seem daunting at first, but it's a very organic process and gets easier and easier. Plus, remember, this is not a system where the monsters need to be strictly balanced (either against each other or the PCs), where monsters have to "follow the rules", or where the system needs to churn out XP values for the monsters or challenge ratings for them either. That gives you a lot of freedom and wiggle room when making monsters; get creative and enjoy it.