Helga's head is basically a circle.
To draw the further elements, it would be helpful to draw some guide lines:
Helga has two eyes (yep!), one of them is "fat" and other one is "skinny". Both eyes "lie" on the horizontal center line.
The fat eye is basically a circle, its diameter is about 1/4 of the head circle. It's positioned at the distance of about 1/4-1/6 of its diameter from the vertical center line (i.e. slightly shifted from the middle to the right). Actually, usually this eye is not a circle, but has a more complex, slightly trianglish shape.
The skinny eye is about the same height as the fat eye, but narrower. Its inner edge is at the same distance from the vertical center line as corresponding part of the fat eye, it's slightly inclined to go the same direction as the vertical center line. The eye's outer edge slightly stands out from the head circle, and the outstanding part of the eye is slightly inclined to go the same direction as the vertical jaw line.
The nose is basically a circle, its diameter is about 2/3 of the eye circle, i.e. about 1/6 of the head circle. It either touches the vertical center line, or slightly overlaps it. The nose circle overlaps the horizontal center line at about 1/3-1/4 of its diameter. Sometimes it does overlap the skinny eye, sometimes it does not.
Helga's nose nearly never is a perfect circle, it usually has some "angles" in it, that make it look slightly squarish. The nose line can start at the vertical center line, at the skinny eye line, or wherever else you want. The nose line ends with a distinctive hook, representing a nostril. Sometimes the nose is slightly longer than pictured here.
The real jaw line usually does not follow the vertical jaw guide line we have drawn before, it has some curves in it (even though, Craig Bartlett himself usually drew it straight). It comes from the skinny eye perpendicularly to it, and then turns to touch the head circle. The general direction of this line should follow the guide line we have drawn before, thus being too close to the head circle in the beginning of the line, should be balanced with the tip of Helga's lip being farther from it.
The ear is basically a circle, its diameter is about 1/2 of the eye, or slightly larger. It's positioned at the distance of about 1/2 of its diameter from the head line; somewhere at 4:00-4:30 hours if the head circle was a clock face.
While the lower edge of the ear comes from the head circle, the upper edge slightly dives into it. Pay attention to the distinctive representation of Helga's auricle.
The end of the ear line is the beginning of the hair line. It ascends in the general vertical direction, passes the midpoint between the fat eye and the upper right part of the head circle, turns to go in the horizontal direction, until it ends with a hook going behind the head circle.
Helga has two pigtails, one of them looks longer (here: the right one) and the other one shorter. The length of Helga pigtails can be different, but usually the longer pigtail is about 2/3-1 of the head circle, while the shorter is about 1/2 of the head circle. The pigtails can be up or down, or take any intermediate position, depending on Helga's mood. The two upper corners of Helga's pigtails usually are at the same level, and the two lower corners are at the same level as well well. The tips of the pigtails are about 1/2 of the head line wide, and are slightly bulging. The bases of the pigtails are about 1/3 of the head circle wide. The lines of the pigtails (both upper and lower) come into the head circle perpendicularly or close to that. The tips of Helga's pigtails are more or less parallel to the head circle from which they grow.
Well, in the end you should get something like this.
You can paint it if you like.