The dimensions are approximately 280x170x30mm (including the bottom part).

It features:

Main pocket and a small front pocket.
Optionally, you can add inner pockets, webbings, etc.
The pattern is designed with an 8mm seam allowance and a 6/8/10mm allowance for a zipper, which can be adjusted by cutting the pattern accordingly.

You can download the pattern and other related files from this link: https://787hsh.github.io/projects.

Supplies

basic sewing tools (scissors, ruler, marking pens,..)
some fabric (preferably softer one for bottom part if youre planning to make it)
zipper coil and sliders
webbing
edge binding tape

Step 1: Printing, Cutting Paper

Print out the required files and carefully cut along the outlined patterns.
Glue together the parts that didn't fit on a single A4 sheet, specifically the back and bottom part.
Cut the small darts at notches to mark the center. Optionally, cut the grey boxes to indicate the position of the webbing.
Trim your preferred seam allowance for the zipper parts. You can determine this by sewing your chosen zipper onto a scrap fabric and measuring the resulting seam allowance. Typically, it's around 10mm, so you can leave it as is. Standard zippers are usually 35+-mm.

Step 2: Marking and Cutting Fabric

Tip: To facilitate easy and accurate cutting, align and mark the front patterns together as illustrated in the first picture.

For the small pocket, mark it, fold it, and then mark the zipper line before making the cut.
When marking and cutting the remaining parts of the pattern, pay attention to details. Don't forget to fold or mirror the bottom part if applicable. To save fabric or utilize scraps, add seam allowance, cut botom part twice, and then sew them together.
Ensure that when cutting the back of the shoulder bag, place the fabric right side down to achieve the correct orientation.

Step 3: Prepare the Shoulder Bag

You will need a hanging system; I utilized a simple 10cm length and 10mm width webbing, melted at the ends. Later, you can insert a rope that will go around the shoulder (this step comes last).
The main zipper measures approximately 27 cm, while the front pocket zipper is 21.5 cm.
Use a 15 cm edge binding tape to topstitch the raw edge of the front pocket.
Remember to cut the centers of the marked patterns, including those for the front pocket, low front, and back.
Optionally, consider adding webbing for the side front part and 8cm zipper pull helpers. Be creative, as there are numerous adjustments you can make in this area to suit your preferences.

Step 4: Making Small Pocket

Tape, glue, or sew the zipper. Topstitch the zipper and repeat on the other side.
Lock the zipper on the left side to prevent it from coming apart and carefully melt the end slightly.
Properly install the slider from left side of the zipper.
Pin down the pattern
Starting from the bottom, topstitch the front pocket with edge binding tape and continue sewing the rest of the pattern.
Reinforce the zipper if you haven't locked it before to prevent it from separating.

Optionally, consider adding a zipper helper for functionality

Step 5: Main Zipper and Top Part

Sew on the zipper using a zipper foot (you can also glue it, but topstitching will be done later for a more visually appealing finish). Repeat the process on the other side with the top part.
Optionally, you can use bias tape on raw edges, but be cautious if your machine can handle sewing through many layers together when topstitching.
Install zipper sliders from both sides, ensuring there's no excessive space when they meet.
Sew on the zipper helper, making sure the seam allowances of the zippers are sewn apart from each other on the wrong side.
Lock the zipper on the other side to secure it.

Step 6: Side Front Piece and Top Stitching

Arrange the patterns together as shown in the first photo, clip them down, and sew with an 8mm seam allowance. Ensure that the seam allowances of the zippers are not facing each other (as shown in the fourth picture).
Optionally, consider binding the raw edge for a cleaner finish.
Once you've sewn the front part of the shoulder bag together, you can start topstitching.
Begin from the bottom and work your way up, but stop near the zipper with the needle inside the fabric. Lift the foot, rotate it, and continue stitching along the side of the main zipper.
Repeat the topstitching on the other side, preferably from right to left at the top.
Sew down the hanging system, ensuring it is positioned at least 1cm from the side (refer to the last step for additional tips).

Step 7: (Optional) Inner Pocket

Fold the back of the shoulder bag along the indicated blue line.
Cut the fabric along the fold.
Bind the edge of the cut fabric.
Sew the pocket onto back

Feel free to get creative with this space and customize the inner pocket according to your preferences or needs.

Step 8: (optional:) Sewing the Bottom of the Bag

Align the center of the back and bottom pieces.
If your fabric is sturdy, consider making ease cuts (cuts on the side of the fabric to increase flexibility, but be careful not to cut beyond the seam allowance).
Clip the pieces together and sew, reducing the stitch length in corners. A helpful tip: start sewing each side from the center to the top; avoid sewing from top to bottom.
Repeat the same process with the front piece of the bag.
Trim any excessive fabric.
Bind the raw edges for a finished look.

Step 9: Finnishing

Pin down the top part, ensuring that the bottom fabric at the corners goes inside, not outside (as shown in the first picture).
Sew it together and cut any excessive fabric and webbings.
Bind the raw edges for a polished finish.
Heat the corners and press them down with your fingers.

With these steps, you've completed the sewing process. Now, it's time to move on to making the shoulder strap.

Step 10: Adding Hanging System

For the shoulder strap, there are numerous possibilities for creating a functional design. In the picture, a simple 5mm rope and hitch knot were used.

One straightforward option is to sew two approximately 40cm webbings at Step 6 and attach a buckle at the end for adjustability.

Feel free to explore various designs for the shoulder strap based on your preferences. If you're interested, check the next step for the usual way of making the hanging system.

Step 11: Webbing + Cord Hanging System

Start by inserting approximately 1 meter of webbing into the female part of the buckle.
Fold the webbing twice and sew it together to create a stopper.
At the end of the webbing, make a hole for a cord, or alternatively, use hardware like a plastic triangle.
Lock stitch the other side of the buckle to prevent it from slipping through and sew the same eye at the end of another piece of approximately 20 cm webbing.
Thread approximately 30 cm of cord through the eyes and tie any bend knot, such as a fisherman's knot.
If you don't have carabiners, thread the cord through the hanging webbing of the shoulder bag.