You need to hire programmers and developers to take on this job. I just don't know how best to do it, since you don't need them on a full-time basis
Why do you need to hire someone for a one-time job? Isn't it easier to outsource the task? You just need to find a reliable software development company website. We addressed this issue here: https://unionsmarttech.com/ In general, I am satisfied with their work, as it was done on time and in exact accordance with the terms of reference.