I realised that pillow cases are really easy to make! Shall briefly jot down the steps for easy reference in future.
1. Measure the existing pillow case. For the case I'm using-->
Exterior: Front-15" by 6.5", Back-14" by 6.5", Interior flap: 3".
2. Add 0.5" allowance on each side, mark and cut the fabric
3. Fold in 0.25" on side of the fabric (where the case flaps are), fold again so that raw seams are hidden nicely, pin and sew. This is how it should look like after sewing, great to prevent fraying.
4. Mark your sewing guideline using tailor's chalk or erasable marker (shown in pink!)
4. Place fabrics on wrong side together. Decide how deep the flap should be. I left mine 3" deep. Pin fabrics to prevent movement while sewing.
5. Sew along the sewing guideline you marked.
6. Serge (or zig zag) the seams to prevent it from fraying.
7. Flip it over. You're done~ :)
The back view looks like this..