Sign [Q&A] Missing Play Store or others after Flashing ROM and Gapps

Mainly caused by not properly following the steps below

1. Boot into CwM or TWRP recovery.
Wipe cache, dalvik cache and data.
2. Flash rom.zip. and Reboot.
Allow tab up to 20 minutes (usually takes much less time) to reboot to the initial Setup screen.
Complete the initial setup (CM Account setup is optional).

3. Go to Settings, and enable Developer Options, allow installation from Unknown Sources,
enable Android Debugging, enable Advanced Reboot, etc. and then Reboot.

4. install GApps.

My experience is that the most important parts are

A. wipe cache, wipe dalvik cache and wipe data, if you are not sure, you can do this every time before flashing any .zip and before booting. It won't hurt to wipe it many times.

B. make sure completed configuring all the setting on step 3, including "allow installation from Unknown Sources", before install GApps.

If you believe any step is not right or not necessary, please let me know.