How can I make a DIY clamp sensor for my power meter project?
For a conventional current transformer, you can refer to Colonel Wm. T. McLyman's book Transformer and Inductor Design Handbook. There is a small section on current transformers. You should be able to find it at the library. 

The principles of current transformers are similar to any other transformer, but the details (flux density, core materials etc.) will likely differ considerably if you want good accuracy. 

Another option would be to wind a Rogowski coil, however the Rogowski coil responds to the derivative of current, so there are certain advantages and disadvantages. It's air core, so no worries about linearity. On the other hand, the signal processing requires integration. 

I would suggest considering making a planar Rogowski coil using prototype PCB technology.