A step by step derivation of Ewald summation can be found in:

Williams, D. E. (1971). Accelerated convergence of crystal-lattice potential sums. Acta Crystallographica Section A, 27(5), 452–455. doi:10.1107/S0567739471000998

Some additional papers that might help you are:

De Leeuw, S. W., Perram, J. W., & Smith, E. R. (1980). Simulation of Electrostatic Systems in Periodic Boundary Conditions. I. Lattice Sums and Dielectric Constants. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 373(1752), 27–56. doi:10.1098/rspa.1980.0135

and

Nijboer, B. R. A., & De Wette, F. W. (1957). On the calculation of lattice sums. Physica, 23(1-5), 309–321. doi:10.1016/S0031-8914(57)92124-9

A step by step derivation of Ewald summation can be found in:

Williams, D. E. (1971). Accelerated convergence of crystal-lattice potential sums. Acta Crystallographica Section A, 27(5), 452–455. doi:10.1107/S0567739471000998

There's also an expanded version of the above paper:

Williams, D. E. (2006). Accelerated convergence treatment of $R^{-n}$ lattice sums. In U. Shmueli (Ed.), International Tables for Crystallography. Volume B. (pp. 385–397). Kluwer Academic Publishers.

Some additional material that might help you:

De Leeuw, S. W., Perram, J. W., & Smith, E. R. (1980). Simulation of Electrostatic Systems in Periodic Boundary Conditions. I. Lattice Sums and Dielectric Constants. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 373(1752), 27–56. doi:10.1098/rspa.1980.0135

Nijboer, B. R. A., & De Wette, F. W. (1957). On the calculation of lattice sums. Physica, 23(1-5), 309–321. doi:10.1016/S0031-8914(57)92124-9

I think between the papers of Williams and the appendices in Nijboer and De Wette, it is possible to fill all the gaps (at least in the case where there's no dipole moment).