Each retailer will have their own interface for you to buy Skype credit; however, the general process will be:
Type in your Skype name.
Confirm your Skype name is correct and continue.
Put in the amount of money (in Russian Roubles) you wish to add to your Skype Credit. Note that your payment will automatically be converted from Roubles to the currency you have selected for your Skype account.
At terminals and retail locations, you will get a printed receipt.
Your Skype account will be credited the amount of money you put in.