To tag a friend on facebook post, you need to follow these steps:
Open facebook app in your smartphone.
At the top, there is a section to put your status or post. Click on it.
Now you can type any message there or place a photo or video etc. 
In this example i am just writing something as shown in below image. After that you need to click on below given icon bar to see the available options.
You will see multiple options. Choose "Tag Friends".
Next you can see the list of friends where you can choose multiple number of names to tag them.
