Start with your apron, just use it as a pattern to cut the fabric the shape you like it.
(I did it a bit bigger than my apron)

After cutting it, I wanted the lower edge to look more damaged.
To do this, just hold the fabric and make some messed-up ruffles, big and small ones, no order at all...

...and just cut-cut-cut.

Here's how the edge came out.

Now take you apron again and measure the length of the side and the neck stripes and mark it.

The nice thing about zombie fashion is that they don't care about rough edges and sawing!

So I measured the stripes with my fingers, 3 fingers in the folded stripe.

Now take the neck stripe and hand-sew it. I did a fold to avoid the sawing of all the edge -zombies are messy!-

Saw also the side stripes.

And ta-da!!
Your messy zombie apron is ready!!

Now, for an extra decoration, you can add a message on it.

Decide on your phrase and write it with a light pencil.
Since I didn't had a brush near me, I took a cotton bud to paint the letters.
It resulted quite well! the letters look very messy, zombie-like.

Ready!
Now is ready to wear, get diry and bloody with nice brains and candy.

Hope you like this idea, very easy and cheap to do, also funny to wear!