It depends on the type of SLM you have. If it is a device which modulates the phase front of the light then you should imprint a phase function resembling a Fresnel Lens which you can see in this wiki article:

https://en.wikipedia.org/wiki/Fresnel_lens.

To create a converging lens using a phase modulating SLM then you would imprint a phase level which decreases quadratically from the centre of the SLM (similar to the way a lens would) or the centre of the incident light you are shining on it. If the phase level goes beyond the range of the SLM you wrap the phase level round to make the function piecewise continuous.

On a side note I did a little bit of googling and found this lecture:

http://www2.ph.ed.ac.uk/~wjh/teaching/mo/slides/lens/lens.pdf

I think it has a pretty good set of worked through models of scalar waves which may be useful to you.

Further, on a side note, you need to make sure that whatever you use to set the phase level to corresponds to a linear scaling of phase as close as possible to a range of [-pi,pi) to get the best results out of the device.

I think this paper of old give a good overview of the various algorithms - good luck :)

Interactive approach to optical tweezers control, J Leach, K Wulff, G Sinclair, P Jordan, J Courtial, L Thomson, G Gibson, K Karunwi, J Cooper, Z J Laczik, M Padgett

 Appl. Opt. 45, 897-903  (2006)

Dear Sahil,

in addition to the previous mentioned paper and lecture, you can add the following material:

-Akondi Vyas et al., "Spatial Light Modulator for wavefront correction", 2009.

-A Reflectance Display: Supplemental Appendix - Harvard University at vision.seas.harvard.edu/reflectance_display/SLM_appendix.pdf