Slide the plastic jacket off of the cube.
With a sharp, non-serrated blade, slit the rockwool deep enough to comfortably accept yer roots.
Hold the edges of the slit you've just cut gently apart (ya don't wanna split the block) and tuck the roots inside- gently, and neatness counts.
If ya have to backfill the slit, or the center hole if yer using that style block, just sacrifice a block to do all the others- tear it apart for fibers.

If yer blocks are damp, ( which they should be from overnight soaking in water adjusted to pH 5.0, which will condition the block to a workable pH,) you'll raise a lot less dust.

If they're well rooted, ya shouldn't have to baby them much- keep the lights high for a couple of days.

Of course, if yer clones are in rapid rooters or rockwool plugs or something like that, just stick it gently in the center hole and yer done- again, shouldn't need much coddling.