How to use Skype on Android 2.3.5 Can anyone explain me how to use Skype app on Android 2.3.5, because it is no longer available on Play Store for this Android version?

Thankful in advance
As already mentioned in comments, get an older apk. Here's a link to one from Apk Mirror, a fairly reputable site for downloading apk's. To install the apk on your device:

First, enable Unknown sources.
Enabling unknown sources allows you install apps outside of the Google Play Store. 
Before you start, you need to make sure you have allowed your device to install apps that are found outside of the Google Play Store. 
To enable installation from these “Unknown sources,” as Google calls them, go into the system settings and scroll down to Security. Inside this menu, you’ll see the Unknown sources option. Check the box, press OK, and you’re good to go.

This is quoted from this site. Visit that site if you want more options for sideloading apk's. But if you downloaded the app from your computer, hook up your phone to your computer, and copy and paste the apk to your phone's files. Then unhook your phone, and find where you put the apk. Then select it, and it will bring up prompts to begin installing it. If you didn't get the apk i directed you too, and it's too new for your phone, it may say "Problem Parsing Package", which means you have downloaded an apk that's non compatible for your device.
 If you install the app and try running it, but it doesn't let you log in, make calls etc, unfortunately that means skype has discontinued its support for devices that old, as mentioned above in comments.