How do I add leaf springs to my van?

If your van already has leaf springs then get the correct replacements from dealer or auto parts store. You will need some helpers, and jacks and maybe special tools, there is some danger due to having to “load” the spring set to get back to the mounting bracket. it may be a job for a shop.
If you are adding leaf springs or “helper” springs you probably need to discuss with mechanic for proper brackets and mounting