how to design hydraulic cylinder?

The easy way (I'm assuming you're an engineer) is to just buy a cheap one from somebody like

www.mcmaster.com

take it apart and study it. In most simple cylinders, all the features are function-driven and, when you see things like the seals, the honed finish, port size, etc. ask your self, "Why is it like that?". When you finish that, talk with four or five potential customers and ask them the value (to them) of the various features. Then take a look at your potential competitors and ask yourself what value you can bring to a customer that is not already provided by somebody else. Then you're ready to design.

Good luck.

I would only ask why do you want to design cylinders when you have a very broad offer of standard and special designs and for every kind of application.

A cylinder gives the impression to be a very simple element and as long as you do not go into details it is but if you start to analyse the application, the life expectancy, the pressure, the velocity and the stroke the design becomes very complex.

The same with manufacturing there are many ways to do cylinders honed or not honed apparently it is simple and in fact complex.

The best council I can give you is as long as you do not have the possibility to invest a lot of money in developing your own know how you will be better with a bought cylinder for which you will only have to write the specification.

There are several books written on cylinders so that it is not possible to give you complete details on a thread.