You will need to start by drafting a pattern.  Don't worry, I'll walk you through it.  The dimensions of this bag are 7" wide x 4" high x 3" deep.  Firstly you need to draw a rectangle 7.5" x 4 3/8ths":

Then you need to add the sides by placing a quilting ruler (or any right angled ruler 1 1/2" out from the bottom and the other side of the ruler against the other side corner, like this:

Repeat for the other side so it will now look like this:

Then add the bottom by adding a rectangle 1.5" x 7.5" to look like this:

Congratulations, you have now finished drafting your pattern!!!  Now, cut two from the main fabric and two from the lining fabric.

If you want to know how I constructed this bag, please read on.  I have a few tips for you too...

Start by cutting two 1" squares.  These will help make your zipper sit neatly into your bag and give you nice corners at the top.  

Sew the first square to the metal crimp end of your zipper (but please don't sew over the metal crimp ), like this:

Open your zipper, then place the other 1" square 6 3/4" away from the other end of the other square, like this:

I know my little end bits don't look square - that's because they're not .  I did this to give myself a little give - I will trim them down later.

Press ends outwards and it will look like the picture below (I trimmed the end bits to 3/4" from the seam just sewn to the end of the fabric):

Sandwich your zipper between one outer piece and one lining piece that have right sides together.  Line the zipper up with the top.  You will have little end bits that stick out from the end of the zipper.  Don't worry about those.

This is a better picture of the little end bits that stick out:

This is what it will look like once you have sewn one side:

Repeat for the other side.  Open out, press and topstitch close to edge.

Firstly open your zip.  This is important or you will have trouble turning later.  Now place the two outer pieces together and two lining pieces together (right sides together) and sew around leaving a gap at the bottom of the lining.  Stop and start at the corners.  When you get to the zipper, pivot at the seam.

Fold each corner and stitch across.  As a tip, either press your seam allowances open or make one seam allowance go one way and one the other way (you can see this in the picture below).  It makes your seams less bulky.

This is what it should look like once you have completed all four corners:

Turn right sides out.  Gently push the corners out of the outer fabric by placing a knitting needle through the gap in the lining and gently pushing the corners out.  Now is a good time to press the bottom corners.  

Sew opening in lining closed.

Voila,  your new straight sided bag: