For the 'best' way I guess that's personal preference. The easiest I assume is standing or sitting because that's how most people pee at the toilet. The way with the least chance to leak: no idea I'll get back to you once I get some diapers to try!
It really just depends on the person. For me when I was first trying to get started wetting, I needed to be standing. It would take awhile of getting used to everything for me to be able to wet while laying down. Now wetting while laying down is very easy for me.
So I can't really say that there is a best way. It varies from person to person.