How do nestboxes work with Pacific Parrotlets (Pocket Parrots)???

Question

We are going to be purchasing a pair of Pacific Parrotlets and ive read all the breeding info etc. but i dont understand how u attach the big brown wooden nestboxes to the side. Plus, how would the birds get in it if its on the outside?? Would i have to bend the bars on my cage so they could get through and into the nestbox??? Please help