Steps 1-2

Step 3 V-fins
[A] This is what we're starting with, and the first step is to clean up the lines we don't need, until we only have that one cross-lines and the general circle for the head.

[B] Cleaned up "head". :3c Now for the V-fins.

[C] When we start adding details to the head, I generally tend to begin with the Vfins, since they'll hide parts of the helmet and save us time erasing. Now Seishin here has a 'rare' Tri-Vfin. But before we can draw the fins, we have to draw the crest that they attach to. This particular gundam has a standard, flat gundam crest. (Note: I've made that horizontal line the line for his visor/helmet edge above his eyes.)

[D] When dealing with Vfins, draw the lowest, longest Vfin first. Take your time, finding the right angle takes some practice, so don't be afraid to erase if you have to. Seishin has a nick on his lowest Vfin, so don't forget any special markings on these. Might as well do them now, since we won't come back to these after we're done until we start inking. When you're done with the lowest Vfin, start working your way up to the highest, smallest one.

[E] Once your done with one side, go ahead and do the other side. Here I've used several horizontal lines to triangulate where the other side's Vfins end to help me with the angles some.

Step 4 Helmet

[F] Alright, now that those pesky, oversized Vfins are done, we can move onto the helmet itself. A good place to start is putting that spare visual sensor in behind the Vfins, and arcing it over to the back of the helmet. Seishin's curls up slightly right at the end, suggesting he has a visual feed on the back as well. Some gundams don't have this, but it's rather typical for the original Gundam series' to have one back there.

[G] Next we draw in the side-vents on the helmet. Remember it's okay to go outside the manniquin, it's just there to help us get our pose and general size down. Again, don't be afraid to erase if its hard to see what you're doing. (To help with this, I usually use a .7 mechanical pencil. A friend of mine uses a .5, and his sketches come out even better [despite the fact he doesn't draw as much mecha as I do!].)

[H] Following the side-vents, we connect the edge of the visor to the forward part of the vent. Some gundams don't have the vent come out this far, so a simple "L"-looking shape serves to solve this, unless the helmet curves away from the gundam's facemask (Like Mack's does). Make sure to add some depth, it really helps make anything you draw come to life. Also, I've added in Seishin's scar in his visor, and the one on the other side is hard to see.

[I] Here we're going to go back a little bit, and put in the spikes that come off of Seishin's helmet. These are a lot like V-fins, so it shouldn't be too hard once you get the angle. One spike comes off the vent itself, but before we can draw the second one, we need to put Seishin's volcans in.

[J] After the second spike is in (directly off the helmet), erase the center line down the face and draw in the closer half of the facemask. Often the far side of the mask is seen as a mere sliver, or not at all, unless the gundam is looking more directly at the viewer. Seishin's mask is pointed, so a group of "V" shapes achieve this look. A flat mask (Like Zero's or Baku's) is similar, but instead of coming to a point, it has a few more lines to get the "flat" part of the mask, practically erasing the implied nose that a pointed mask has.

[K] Finish off the mask and the far side of the helmet. Don't forget to leave space for the optics, unless the angle only allows for one optic to be visible. Add in the beard/gaotee, and any other facial markings. Seishin has a double-fang marking on the upper and lower part of his facemask, but we can't see the lower one at this angle.

[L] Draw in the optics and the ovals for the eyes (Unless the gundam doesn't have eyes, but Seishin here does. I've left it out because... I briefly spaced out. Px they'll be in there in Step 6... Maybe. Unless I change my mind and want a different look for his expression.) Shade in the recess behind the optics above the mask, and finish shaping up the head. Don't worry too much about minor adjustments, that can be done when you ink when you're done sketching.
~~ Remember, while eyes are certainly expressive, optics can emphizise expressions like anger and astonishment just as good, and even better when combined with the eyes. Here I've narrowed Seishin's optics to show either focus or anger, and only used one optic visible since the second took away from the single "glaring" look the single optic has.

Step 6