how to start to develop customized development board??

Before you go build your own custom board, you have to ask what use(s) do you plan for it. Once you decide the goal, then ask why a current board does not fit your needs. It is possible to build a board, but it takes much time and skills. For a one off ask, is it worth it?

So here is a good read about one persons attempt at designing a board. I do not suggest this is the only way, just a plausible one. The skills are being able to soldier and planning with much use of the internet. If you can elaborate more, Maybe I can give more specific examples. Hope this helps.

I'm all for electronics, so please don't let my next comment put you off, but I agree with clem57 comment - is it worth it? There are so many great dev boards avaiable at very reasonable cost.

What would be the goals of designing your own? A few answers could perhaps be:

•           To have met the challenge of being able to do so. e.g. for fun
•           To understand, again because you just want to.
•           To build one that has very specific specification - peripherals or battery life .
•           To mass produce one for market. (I think this was previously discussed)...e.g. to make money.
•           To make a locally available board - with suitable PCB stencil in local language.

Out of interest shakti334 who are you referring to by the following statement - a buisiness company, a maker group, some friends or a school/university group ?

But how can we design...

If you did want to take this route it very much depends what skills you have and which are available. Personally if making myself I would use the tools and manufacturers I was most familiar with.

Therefore I would already know that my board would utilise one of the Microchip MCU devices as I am experienced in using those. I would think about what specification I needed on the board and identify some suitable device familes. I would look more closely at the footprints and costs if it were for mass production and eventually select a model to use. I'd do lots of reading of the datasheets. I'd identify other circuit components required such as a buffer, USB-UART bridge, power supply, programming header, and whatever else my specification needed. I would identify the actual components required and then start making the PCB design. For that I would use Autodesk EAGLE because I am also familiar with that. Others would likely use Altium Designer or one of the other PCB packages. To have the 'Arduino experience' of a managed boot up you would also need to start writing some boot code (which would also setup and run the UART/USB). You'd likely need a few test boards to rigorously check for operation.

If you were marketing this you would have lots of other things to do. For example local regulations to comply with - RF interference, component specifcations, electrical safety. You would need documentation, a website ready for startup and tutorials. You would need packaging, branding,advertising and customer support.....lots to do and it nedds doing those parts well to make it successful. Finally after all that effort you would find things are moving on and so this would need to be a quick process and one you find you have to repeat quite often. If that is still your aim then I wish you well

As the others have said, you should define your objectives and needs.  But if you are interested in what the general steps are there is a tutorial here for creating a small Microchip (was Atmel) development board using KiCad.  I have used KiCad to design boards when I wanted to use a microcontroller where a dev board wasn't available or I had a enclosure it needed to fit.  Expect to spend time developing the skills but it can be done. 

That's a great link.

If you really want to design your own development board, then I agree with the posters above. There's not a great deal of point. However, I suspect what you really mean is you want to design your own custom board with a microcontroller on it. This is really worthwhile.

I've made a few of my own boards based on the MSP430 microcontroller. Here are a few tips to get you started:

• Learn from your development board.
  There will be a few essential components on there such as decoupling capacitors, pull-up resistors, etc. Take parts of the design that have been done by someone who really knows what they're doing. They probably had a good reason for adding each component. Then double-check with the datasheet.
• Add a debug header.
  You probably won't want the debugger on your board, so make sure you have an appropriate separate debugger and add a header to connect it.
• Add a few LEDs or even a serial port for debugging.
  Always handy, even if the final design doesn't need them.
• Pick a package for designing your board.
  I chose Eagle, but there are other options. It will take a while to learn the package, but you have to do it. Find tutorials online.
• Separate parts of your layout with traces that are easy to cut.
  Assume that you will make mistakes and some parts of the board won't work. If you've made a mistake with a sensor it will at least allow you to test that the microcontroller part works. You can check each bit in isolation. That way when you spin up version 2.0 you are more likely to have found most of the mistakes.

I hope some of these tips help.