Good day to you!
In this article I will tell you, how to backup all the data from an Android device (a phone or a tablet) properly in order to restore this data after cross-flashing or on a new device.
I treat the following data as important: list of contacts, SMS, photos and videos and, to some extent, applications.
How to backup data from Android device
How to backup contacts on Android:
One of Android operating system advantages is that all the contacts are attached to Google account by default and stored in a cloud. That is, as soon as you type in your Google account data on a new device (or on a cross-flashed one), all your contacts will be immediately downloaded and become available.
To be on a safe side, let us make certain that your contacts do really synchronize with Google cloud. To do this, enter your mail box via web, following the address https://mail.google.com/
1. Type in your Google account username and password.
2. Click the top right button “All applications”:

3. In the appeared list of applications, select “Contacts”.
4. You will see all your contacts in a new window. If there are no contacts there, go to your Android device settings and enable Google account synchronization (All applications -> Settings -> Accounts -> Google).
How to backup SMS on Android:
Unfortunately, there is no in-built application for SMS backup in Android system, however, there is a great option for that – SMS Backup & Restore application. Download it from official Google app store and run it (it’s free):

Open the application, press “Backup” button, leave all the settings as on default and click OK:

The program has saved all the SMS and calls data to the special files in the internal memory. Just to be safe, we will copy these files to the computer together with photos and videos. Read further on the matter.
How to save photos and videos from Android device:
1. Connect your phone to the computer, using USB cord, with the phone unlocked.
2. Now proceed to Windows Explorer (My Computer, This Computer) on your computer, you will see the connected device:

3. Get in this device and copy the following folders from it to the computer:
- The whole DCIM folder, it contains your photos and videos;
- The whole Download folder, it contains your files, downloaded from the Internet;
- SMSBackupRestore folder, this folder contains your previously saved data on all the SMS and calls;
Hereon, I will complete the instruction. If you need to save the data of a certain application, please, ask a question and I will tell you how. Unfortunately, there is no universally applicable instruction for saving these applications.
The instruction is not detailed enough in some parts, if you need further details – just ask a question.
Good luck!