The "soft nair" means that you use the weaker hitbox to get less knockback, so they are still in rest range. I'd recommend looking at a frame data thread to get yourself a picture when which hitboxes are "soft" nairs, because seeing it yourself is often better than just getting explained.
You shouldn't use up-B in the combo. Crouch is somehow optional, it doesn't really change anything (people use it because down is already pressed then and they only have to press B in the right moment). Just position yourself in a way that you are exactly there where the nair will hit them
There are multiple nair->rests in
this video, just to learn a few setups: For P:M setups, most work the same,
Hungrybox vs Oracle had some nair->rest setups iirc.
One little addition, if you see they do everything to avoid the rest after the nair, this often makes nair->grab easier.
Different setups work well on different characters, generally, it is hard to air combo nair->rest against fastfallers, so against them I would rather use setups near the ground. Use different hitboxes and timings depending on the percentage if you see it doesn't work with your standard method. Try out different things and see what works.
