I have made a short step by step guide for burning a bootloader into the ATtiny3217. 

The ATtiny3217 is a very cheap and small Arduino compatible processor with 21 I/O lines. I have made a small development board: ATtiny3217 development board.

Read the article about the megaAVR ATtiny family

I want to thank Spence Konde for writing an extensive article about the megaAVR ATtiny, see GitHub.

Required

• ATtiny3217
• Arduino Nano
• 10uF capacitor
• 4.7k resistor
• FTDI adapter

Step 1

Making an UPDI programmer with the Arduino Nano.

• Tools -> Board = Nano
• Tools -> Atmega328 (sometimes old bootloader)
• Upload the jtag2updi sketch to the Arduino Nano see GitHub 

Step 2

Connect the 10uF capacitor and 4.7k resistor, see the image here. 
The UPDI programmer is now ready

Step 3

Installing the megaTinyCore package.

• File -> Preferences> Additional board manager URLs = http://drazzy.com/package_drazzy.com_index.json
• Tools -> Boards -> Boards Manager ...
• Select "megaTinyCore by Spence Konde" and click "Install".

See here an Arduino Nano used as UPDI programmer. If you only use this Arduino Nano now as a UPDI programmer, you can skip the steps above the next time.

Here the combined FTDI / UPDI program connector is used:

Step 4

Testing if the UPDI programmer works properly.

• Open the sketch ATtiny3217-blinking-LED, see below
• Tools -> Board = Attiny3217 (without optiboot!)
• Tools -> Programmer = jtag2updi (megaTinyCore)
• Select Tools -> Port
• Upload the sketch and check if the LED is blinking

The LED will continue to flash now.

Step 5

Burning the ATtiny3217 bootloader, after this, UPDI is not working anymore.

• Tools -> Board = ATtiny3217 (optiboot)
• Tools -> UPDI / reset pin = reset (Danger)
• Tools -> burn bootloader

The bootloader is now burned into the ATtiny3217.

Step 6

Testing if the bootloader works properly and if we can upload sketches via the serial pins RX and TX.

• Disconnect the UPDI programmer we made in step 1
• Connect an FTDI adapter between the PC and the ATtiny3217
• Tools -> Board = ATtiny 3217 (optiboot)
• Upload the ATtiny3217-blinking-LED sketch and check if the LED is blinking.

ATtiny3217-blinking-LED sketch

// File -> Preferences > Additional board manager URLs = http://drazzy.com/package_drazzy.com_index.json
const byte ledPin = 3;

void setup() 
{ pinMode(ledPin, OUTPUT);
}

void loop()
{ digitalWrite(ledPin, 0); 
  delay(500);
  digitalWrite(ledPin, 1); 
  delay(500);
}