A cow has a simple shape, but sometimes drawing one is more difficult than it looks. So rather than draw the whole cow right away, start with the head. The best way to draw a cow's head is to imagine you are drawing the cow inside of a box. Make the cow have exaggerated features for a more cartoon appearance, or make them smaller for a more realistic drawing. After drawing you can colour the cow or paint over it.
Draw a square about the same size that you want for the cow's head. Draw a faint X shape inside the box from corner to corner.
Draw the cow's eye near the centre of the X, along the top right tier. Make the eye about one-tenth the total face size. Draw a line dividing the top and left X pieces in half diagonally. This is the cow's nose.
Bring the line in rounded near the bottom of the square and back to the bottom right corner of the X. Make a rounded jaw line around the right side of the X and extend an ear about 3 or 4 inches beyond the right side of the square, depending on how large the square is.
Erase the square guidelines. Add in details, such as fur, nostrils, shading, highlighting and patterns. Draw two diagonal lines coming down from the cow's head on the right side to make a neck. Build the rest of the body from there, or simply leave the drawing of the head only.
Add in additional shading and details as necessary. Give the cow a pair of horns if you like, to make it look more realistic, or to change the breed of cow.
You can also make the cow face the other direction by reversing all of the directions.