https://www.narita-airport.jp/en/map?terminal=1&map=3


From the gate, you go up one level (escalator or elevator).
Then you have to travel (walk or sections of travellator) to the transit security


Once you pass security, you take escalator down to the main airside area

Depending on time of day, can be a long line. There should a priority line for *G/J, though unsure if that priority line is operating

Edit:jib71 has a better map above of the transit security area