Components List:

Name	Q	Sample Picture	Cost	Comments
Phone Connector	1		$5	Can be hard to get locally. You can order it online from: -PC-Mobile (Hong Kong/UK) -Actioncode-GSM (Yugoslavia) -Handystecker (Germany) -GSM-Technology (Poland) -IT GSM (France)
Serial DB9 Connector	1		$3	Fairly easy to get. However, I recommend getting a cheap or old serial cable, cutting the other end and using the existing wires.
1N4148 Diode	2		8¢	Pay attention to the direction. The side marked by black should be on the side directed by the arrow. These diodes' output should be less than 100mA.
3V3 Zener Diode	2		8¢	Same as above.
78M05/78L05 Voltage Regulator	1		80¢	Any other 5V voltage regulator should work. From the top view of 78M05 with legs faced to you, the left leg is 'Input', the middle is 'Ground' and the right is 'Output'. The 78L05 is just the opposite. You need to use this part to direct power from the COM port to the phone and block the other way. A battery and a lightbulb are useful for testing the part and determining which pin is for input and which is for output.
MAX232 Integrated Circuit	1		$3	This is the core of the cable, so be careful about connecting it correctly. You can use other variations of this part with some alterations. For example, MAX233 requires no 1 uF capacitors, while MAX232a works with 0.1 uF capacitors. Big versions are easier to deal with, so don't get small chips if you have the choice to get big ones.
1 uF Capacitor	4		20¢	Most capacitors have one leg marked with '-' sign. Make sure that you are connecting the capacitor in the right direction. You need capacitors that work under a voltage of 8-12V. Temperature isn't important.
4.7 uF Capacitor	2		20¢	Same as above.
100 nF Capacitor	1		15¢	Small capacitors like this one do not usually have a negative leg and positive leg, so the direction doesn't matter. This capacitor is not essential as my cable worked without it, but it can make the cable more stable.

Circuit Diagram:

 

The whole process usually takes 3-7 hours, depending on your skill and equipment. The parts should cost about $12. A ready cable doesn't cost much more, considering the difference in quality & reliability and the time & effort you'll spend. Unless you are skilled enough and have time and patience, buy a ready cable.

Start with connecting wires to MAX232 pins. Other components should be connected to the wires, not the pins directly. It's better to use colored wires from the beginning as this makes telling them apart easier.  Many people will prefer soldering, but if you miss the skill or equipment, you can try this trick. Get thin insulated wires and a sewing pin. Pass the pin through each wire 5 mm before the end to make a small hole. Now, flip the MAX232 upside down and insert the wires in its pins. Secure them together with a small piece of adhesive tape, and stick them to the base plate.

Pin 1 on MAX232 -and other integrated circuits in general- is marked by a small hole. After connecting the wires to MAX232, connect the 1 uF capacitors to them. Small capacitors can be soldered to pins directly and placed over the chip, but you need skill to do this. After you finish with the capacitors, connect the direct wires. You should finish connecting everything to MAX232 before going to the next step. I suggest printing the diagram in a light gray color, so you can mark the done parts of it with a pen.

Work around the voltage regulator now. Connect the wires and capacitors to it as shown. Complete all connections between MAX232 and the regulator, and add the diodes at both ends of the circuit. Now, prepare the rest of wires used to connect the phone and the serial port. There should be 3 at the phone side, and 7 at the serial port side.

Connecting wires to the serial port should be easy. As for the phone, you'll need to solder long wires to the connector and connect the other ends of them to the wires on the circuit.  Be careful about the connector. Soldering is preferred there, because the wires can get shorted together wrongly by the slightest move. It's best to close the connecter's wires hole with glue after connecting them.

You can place a switch between data and service modes, which simply shorts 3 to 7 (data), or 3 to 9 (service). For flashing the phone, use service mode. For other purposes, use data mode.

Double-check that everything is connected correctly before trying the cable. When you plug a data cable into the phone -regadless of what's plugged into the other end-, the phone will beep after 5-10 seconds (service cables don't make it beep). This doesn't necessarily mean that the cable works, because shorting pin 3 to pin 7 on the phone connector makes it also beep. But if it doesn't beep at all, the cable doesn't work for sure. The fastest way to test the cable with is using All Phones Data Suite (APDS). Set it to connect at startup, and run it after each try. Mistakes are easy to make, but hard to discover. If your cable doesn't work, check...

-The type of wires you're using
-The direction of capacitors and diodes
-The phone connector's contact with the phone
-The type of MAX232 used
-Shorting of pins on the phone connector
-Direction of the voltage regulator.

Remember : The fault is always found in the last place you search for it in :)

Pin-Outs

1- GND: Represents the digital interface and charger return courrent.
2- Charge: Represents the positive contact for the charging functionality.
3- Gen I/O : Connected to a general I/O pin on the baseband digital chip.
4- Power Out : Has a different functionality with different accessories.
5- RX (In) : Part of RS232 Interface.
6- TX (Out) : Part of RS232 Interface.
7- ID : Part of RS232 Interface. Shows the presence of an accessory and is also an analog ID for the accessory. Accessories which provide charger functionality only do not support this pin and their presence is detected with a valid charger voltage.
8- RTS : Part of RS232 Interface.
9- CTS : Part of RS232 Interface.
10- Audio Out : Single ended output delivers 0.8 Vpp into a 1kOhm load.
11- Audio In : Single ended input with a maximum sensitivity of 365 mVrms.
12- Audio GND.

Wires colors inside an original Sony cable

1- Black
2- Red (not connected)
3- Grey
5- Yellow
6- Orange
7- Green
8- Purple
9- Blue