Well, first you have to decide what language and format you want to develop it for, find a recording list, record the samples, and then label the samples in a program made to make oto.ini files (typically setparam).
The simplest language to develop is Japanese, with the simplest method being CV. You can find a recording list on many places, but
this site has many good ones. For otoing, I'd recommend searching for dedicated guides on this site or on sites like YouTube. You can also pay others to do so, it tends to be cheap and it's a good option if you're too lazy or find it too complicated.