How to add widgets to main screen?
Method 1. In default launcher you can setup widgets by holding left button.
Method 2. Two fingers pinch on main screen – access to widgets and icons.
How to make screenshot on MIUIv7?
“MENU” button + Volume – or move the nav bar down and find “Screenshot” button.
How to remove vibration when I enter text?
Settings > Other settings > Language & Input > Google Keyboard > Additional Settings > Here you’ll find the option
How to remove the countdown which shows every time you try to install an application on a memory card?
You need to allow installation from unknown sources in the privacy settings.
Settings > Other Settings > Privacy > Unknown sources (enable)
How to enable installation of non-Market apps?
Settings > Other Settings > Privacy > Unknown sources (enable)
How can I change fonts?
Themes > Details > Font
How to disable the lock screen?
Settings > Developer Settings > Activate “Disable lock screen”
Can I change the screen timeout?

Settings > Screen lock
How to enable screen lock with flip?
You need to install “Gravity Screen” app.
How to display download speed icon in the top bar?
Enter Settings > Notifications > Network Speed
How to display the percentage of battery power in the notification area?

Settings > Other Settings > Battery and performance > Battery Indicator
How to enable slow-mo video capturing?

First enable HD quality, turn your device in vertical position and swipe to the right. You’ll see slow-mo option.
How to use my own music as a phone call?

Settings > Sound > Ringtone Sound > Here you’ll see list of default ringtones. In the bottom there should be “Browse” round icon. Press on it, choose SD Card, find your tune and put a mark. Ok that’s it!
How to enable data access from second SIM card?

Settings > SIM-cards and mobile networks > Internet > Select right SIM card
How to edit the service buttons?

Settings > Notifications
How do I achieve that desktop background picture is also moved in the appropriate direction with screen swipes, and not stood locked?

You need to disable wallpaper scrolling in the settings. Setting > Wallpapers > Select background > “Enable scroll” in the top right corner.
My phone doesn't see contacts from SIM Card.

Well, that’s pretty simple. Contacts > Settings > Display > SIM-card
How do I enter into the engineering menu?

How to control other appliances & and electronics via IR-port?

I would recommend ZaZaRemote. There’s also proprietary app called MI Remote (not in all firmware versions).
I can't download nothing using the mobile internet (for eg Google Play).

Downloads > Click on the three dots (…) at the top bar > Settings > Download size limit > Unlimited.