C-Rex is very good at building chicken-walker type mechas but not as good at humanoid type mechas, not saying that Rex isn't an excellent builder. I can give a few pointers for building humanoid type mechas. First look at my example: http://www.mocpages.com/moc.php/102733
Proportion and joint construction are the most important things, everything else is up to your own discretion. I usually start with building the hips. One stud wide for micro-scale, two stud wide for full scale. Using a 1x1x1 modified brick with the hole in the center and a pneumatic T-piece is the joint to go with. Or if you don't have the T-piece, use a modified 1x2 plate with the hinge.
Head construction is very tricky, and may take a lot of experimenting. I can't offer much advice on this, it is just a difficult topic. My suggestion is to try to make the head detailed in the smallest space necessary.
Another important consideration is to make the mecha flexible enough by making enough joints. The human figure has many more and more flexible joints than can be replicated with lego, so focus on the important joints. These include the hips, knees, ankles, shoulders, and elbows. All of these joints should be able to flex in at least two dimensions with the exceptions of the knees and elbows. The hip joints should be able to flex in 3 dimensions.
Proportion may seem simple but it is easy to mess up. A few things I noticed is that beginners often space out the legs too much, build the arms or legs too long, or make one body part seem too large in proportion to the overall body. A good way to master this is to build a basic frame that has all the joints and the overall body shape. Then the details can be added on later.
One less important topic that I will touch upon is that beginning mecha builders often leave too many studs visible. SNOT (studs not on top) building is very useful to get around this if you do not have many flat tiles or if you have tiles to spare then cover up studs with tiles.
Once you have mastered these basics, be creative with your construction! Don't be afraid to experiment with different shapes. I hope this helps. Permalink