This easy backpack is simple
I sewed mine by hand which took 2 days

Step 1: Materials

Fabric

Strong fabric (I used jeans)

Button

Zipper or string

Scissor

Needle and thread

Measurements of dogs comfortable length and width

Plastic buckle or strip of stretchy fabric

Step 2: Making the Backpack

See the photos above for template of the backpack
Turn over the back fabric piece and the side fabric piece inside out
Sew the other side piece to the back piece
Sew the bottom inside out again
Make sure the bottom lines up with the back piece
Sew the bottom to the sides bottom
You do not have to sew inside out
Sew the other side bottom to the bottom edge together
Line the front piece with the side pieces and sew together
Sew together the bottom of the front to the bottom piece
You will get a box like shape

Step 3: Adding the Features

Take a square pice of fabric and attach the end of it to the back
Sew together with a running stitch
Once the flap end is sewed place the other end of the flap on the front
Mark where the button is placed on the front fabric over the flap
Sew the button on the front
Take the flap and place over the button
Mark and cut a slit

Make holes with the scissor on the top of the front backpack
Tie the end of the string together and place through the holes
Make sure the string ends up outside the holes when finished
Cut the string and start again on the other side

If you are using a zipper, sew the zipper on the top of the backpack

Step 4: Adding the Buckle/strip

Measure the tummy of the dog with the buckle or strip
Sew the strip or buckle to the backpack edge
Fill the backpack with food and water for your dog
Tie the string and button the flap