verb / noun
insert
in-SERT/ɪnˈsɜːrt/
To put something into something else, or a piece put in this way.
She inserts the envelope halfway into the narrow slot.

Etymology
Joining something “in” underlies the Latin sense “to graft or implant”: placing something into an existing whole. The English verb uses that same basic image for putting something into or between other things.
In context
… definitely have enough space here to insert the elastic of course i did leave little gaps where is my gap oh here's a gap so i can put the elastic in