In Jacob Lurie's Higher Topos Theory, Section 6.5.3, he briefly mentions that to stackify a presheaf of $n$groupoids, one needs to apply the "+"construction $\left(n+1\right)$ times, and in general, for a presheaf of $\infty$groupoids, one needs to apply a transfinite iteration. However, not much detail is given about this. Does anyone know where I can read more about this? Thanks.

Thomas Nikolaus and Christoph Schweigert discuss the +construction for $n=2$ in their paper Equivariance in Higher Geometry. They split it up into two steps (I think): first producing a pre2stack out of a presheaf of 2groupoids, and then making it a 2stack. Applied to the pre2stack obtained by delooping the monoidal stack of principal $U(1)$bundles, one gets exactly the definition of a bundle gerbe. 

