Step 1: Materials

Bag Elements:

.5 meters of fabric or so (depending on how big you want it)
4 medium sized snaps (about 1/4-1/2 inch diameter)
1 zipper about the same length as the desired length of your bag (can be cut down easily)

Sewing Supplies

Scissors (zig zag preferred to avoid edges fraying but also some regular ones for cutting threads etc)
Thread the same color as your fabric
Hand sewing needle (or sewing machine)
A solid/non flexible ruler
Fabric chalk
Fabric pins

Decisions before making this bag:

How big do you want the bag to be? (It's a cube basically)
Type of strap/where you want it to hang from: wristlet, crossbody, elbow (probably dependent on size of the bag)

Step 2: Cutting the Fabric

Iron the fabric.
Fold the fabric in half with the right sides (the sides with the clearer pattern or fabric texture) facing each other on the inside
Decide the size of the bag you want and add 1" to the length and height measurements.
Mark a square that size on the fabric at the folded edge.
Cut the square out and cut along the folded edge last to separate the two squares.
Use one square as the guide and mark two more squares on top of the folded fabric.
Cut these out to give you 6 squares.
Decide the length and width of the strap you want.
Add 1" to the length that you want and for the width, add .5-1" to the width that you want depending on the desired width.
*remember you will have to be able to turn this inside out at the end so don't make it too small.
Fold the fabric in half so there is an edge as long as you want your length.
Iron that edge
Cut the width of the strap + your added length.

Step 3: Attaching a Zipper

Take one square of fabric and cut a slit diagonally about 1" in from each of the corners (this can be done with straight scissors).
Place the fabric right side up and lay the zipper upside down so the right edge of the zipper lines up along the left side of the slit (it does not matter which side you make left and right. just make sure the zipper is on one half of the fabric diagonally and does not cross over the slit)
Pin the zipper in the middle of the left side.
Sew up the middle of the left side of the zipper.
Fold the zipper along the seam you just sewed and iron so it is facing up with the fabric.
Take the right/not sewn side of the zipper and flip it upside down.
Line the edge of the unsown side of the zipper along the unsown side of the diagonal cut.
Pin through the middle of the unsown zipper and fabric.
Sew up the center where you just pinned.
Lay the fabric flat and iron.
Stitch back and forth across the zipper at its top and bottom to keep the zip from going underneath the piece of fabric.
Cut the excess zipper part off if necessary.
You attached the zipper to the top!

Step 4: Making the Sides and Bottom

Take two of the five remaining squares of fabric.
Line them up on top of each other so the right sides of the fabric are facing inwards.
Pin one edge of the fabric .5" from the edge.
Sew along the pins.
Take a third square of fabric (this will be your base), and place it on top of either of the two squares sewn together with the right sides facing inward.
Starting at one corner of the line you just sewed, pin the new square of fabric to the piece below it .5" from the edge of the fabric.
Sew this line.
Pin and sew the second side piece onto this third piece (the base piece) with the same .5" seam allowance as the other pieces.
It should resemble one corner of a cube when done.
Add the 4th piece of fabric and sew it along one edge of the base and one edge of one of the sides.
Add the 5th and final square of fabric and sew along the three edges to connect it to the base and two of the sides.
Flip the pieces inside out and fold and iron along each seam to make crisp edges.

*make sure the sides of your bag have the pattern/texture all going in the same direction
**sew in one direction from the inside to the outside of the piece to avoid accidentally having any extra amounts of fabric on one side (this is hand done and fabric is a lose material that shifts a lot)

Step 5: The Strap

Fold the width of the strap in half with the right sides facing inwards and iron along the length of the folded side.
On the open part of the length and one of the width sides, pin .25-.5" from the edge.
Sew one of the the short width edges and then the long width edge
*remember to always sew in one continuous direction so start at the short edge you sewed and work away from it to the open end.
Turn the fabric inside out via the side left open.
Fold the edges of the open side inwards .25-.5", iron the folds, and place a pin to keep the four layers together.
Use a ladder stitch (shown below), to close the open end.
Iron the strap.

Step 6: Attaching the Strap (step 5.5)

Before you sew the top on, you have to pin the strap to the sides of the fabric you sewed earlier.
Make sure the bag body is inside out with the right sides all facing inward towards each other.
If it is a wristlet, pin both pieces to one corner of the wrong side of the fabric so about 1-1.5" is outside the bag, leave the remaining strap length inside the bag.

If it's a longer strap, pin the strap at opposite corners with about 1-1.5" inches hanging above the top edges. Leave the remaining strap length inside of the bag.

Step 7: Adding the Top

Take the square of fabric with the zipper and lay it, right side in, on top of one of the side pieces (also right side in).
Pin .5" from the edge and sew along this side.
Repeat this for each of the 3 remaining sides.
*remember to work either clockwise or counter clockwise to avoid gaps in the fabric.
Turn inside out through the zipper.
Iron the top 4 edges with the right sides facing outward.

*I recommend unzipping the zipper before you do these steps because it's a bit harder to do it once all four sides are sewn to it.

Step 8: Adding the Snaps

With the bag turned right side out, rotate the top square 90* clockwise from the base square creating a twist in the sides of the fabric.
Lay the top piece down so the corners line up with the new corner below it on the base of the bag.
Iron the bag while it is laying flat like this.
Mark in each of the four corners a spot where they overlap for adding the snaps.
Sew the female sides of the snaps to the top four corners of the bag.
Sew the male sides of the snaps to the bottom four corners of the bag, ensuring they line up and can snap to their mates on the top corners.

And now you've made your own bag!