References
Look up on Google (or whatever search engine you use) and check out some real life Tiki masks. Observe the shape, structure of the drawn figure, colors and expression.
Based on these attributes, we will create our own design.
So let's start with a paper sketch. You can even copy a real life mask, as long as the drawing will be just for practice. Remember, you only want to learn how to create it and further improve the learned technique, not copy one's work and brag about it.
Here's my paper sketch. I've included the full-size scan in the .zip file, so feel free to use it if you like.
STEP 1
Go to File > New... (CTRL+N) to create a new document. Set size to 600x600 px in RGB color mode.
STEP 2
Click-drag the "sketch.jpg" file into our canvas. Hit the "Embed" button on the top menu of your screen. Then go to Window > Transparency and set Opacity to 50%. Lock this layer - click the empty square in the Layers palette (F7).
STEP 3
Create a new layer ("Add New Layer" button in Layers palette) and select it. Toggle Rulers (CTRL+R) and click-drag a vertical guide as shown below. Try placing it as axis for our sketch.
STEP 4
Grab the Pen Tool (P) and trace only the external outline. When you're done, go to the Brushes palette (Window > Brushes) and click one of the default brushes, Adjust thickness similar to mine. Just observe proportions between the size of the sketch and stroke size.
STEP 5
Outline the rest of the sketch carefully, to obtain a smooth line.
STEP 6
When you are done, select all (CTRL+A), go to Object > Group (CTRL+G), copy it (CTRL+C) and paste it to front (CTRL+F). Then press CTRL+SHIFT+T and reflect it vertically. Hit OK.
STEP 7
Grab the Selection Tool (V) and place the copied group like below. I've colored it differently to make it easier to notice, so there's no need to change any colors.
STEP 8
Select all artwork (CTRL+A), hold SHIFT key and deselect the thick outline. Go to Object > Expand Appearance. Then go to Object > Path > Clean up.
STEP 9
Select the outer strokes (the two thick ones) and go to Pathfinder window (Window > Pathfinder) and click the "Merge" button.
STEP 10
Copy the resulting shape (CTRL+C) and paste it to back (CTRL+B). Grab the Selection Tool (V) and make them larger. Hold ALT+SHIFT keys to preserve proportions and resize it with centerpoint. Make it look like below.
STEP 11
Select all artwork (CTRL+A). Go to Object > Expand Appearance. Then go to Object > Path > Clean up.
STEP 12
Go to Window > Layers to open the Layers palette (F7) and click the "Add New Layer" button . Name it "base".
STEP 13
Repeat STEP 12 to create new layers: "eyes", "lips", "nose", "upper jaw", "lower jaw". We will create every element on a separate layer, because it will be difficult to make selections.
STEP 14
Select the bottom-most layer, grab the Rectangle Tool (M) and draw a rectangle over the entire artwork. Make sure you remove the Stroke and leave only Fill.
STEP 15
Go to Object > Arrange > Send To Back (CTRL+SHIFT+[ ). Select all artwork (CTRL+A), go to Pathfinder window (Window > Pathfinder) and click the "Merge" button.
STEP 16
Grab the Direct Selection Tool (A) and select any of the remaining rectangle outside our Tiki outline. Hit Delete.
STEP 17
Go to Object > Ungroup (CTRL+SHIFT+G). Grab the Selection Tool (V), select the filling of the Tiki outline and click-drag the small square to the respective layer that the fills represent.
STEP 18
Create a layer above all layers, select outline and move it on this layer. Name it "outline". Then lock it - click on the empty square near the "eye".
STEP 19
From the Layers palette (F7), select the "base" layer. Grab the Rectangle Tool (M) and draw a rectangle similar to mine.
STEP 20
Open the Gradient window (Window > Gradient), Grab two colors of
the same tint, but different tones (such as light yellow and brown) and
add them to the gradient - drag them on the gradient bar. This will be
tiresome work, so grab yourself a cup of coffee 
.
STEP 21
Select the rectangle created in STEP 19 and click the large gradient thumbnail. Make sure the gradient is set on "Radial".
STEP 22
Grab the Selection Tool (V) and shrink the rectangle. Then rotate it 90 degrees. Hold SHIFT key to do an orthogonal rotation. Lock all the other layers.
STEP 23
ALT+click-drag on the rectangle to make a copy. Extend it like below and place it over the filling of the Tiki mask. Duplicate more rectangles till you cover the entire shape.
STEP 24
Select all these rectangles and go to Object > Arrange > Send To Back (CTRL+SHIFT+[ ). Select all the shapes in the layer using the Selection Tool (V) and right-click > Make Clipping Mask.
STEP 25
Copy the original rectangle (the one with the gradient) (CTRL+C), unlock the layer above your current layer and paste it to front (CTRL+F). Repeat STEPS 23-24 for the other shapes. For some areas there's no need to texture it, since they will be too small, so just change the color of the fills to a color you like.
Feel free to resize the rectangle in each layer, to create the sensation of perspective and depth.
STEP 26
To change colors, we must select each element and give it a different
color scheme.
Leave a single layer unlocked and click the small gray circle on the
right side of the layer thumb.
Click the "Recolor Artwork" button on the top menu bar, click "Edit",
then click the "broken chain" button and play around with the colors.
STEP 27
Do the same for each layer. To create some variations, select all artwork using the Selection Tool (V), ALT+click-drag to make a copy and click the "Recolor Artwork" button. As in the previous step, play around with the colors til you get a result you like. Here are a couple of my variations.
STEP 28
Grab the Magic Wand Tool (Y), hit Enter and set Tolerance value to "0". Click on the outline of the Tiki mask (the outline is on the top-most layer, so make sure that layer is unlocked) and change its color to a dark tone of the dominant color in the design. The drawing will have more warmth and the outline will blend nicely with the fill and texturing.
Also remember that Tiki masks are tribal, so use vibrant colors, like it would be painted with natural pigments.
