And then, put the winded 120 roll film and the spool of a 620 roll film into a changing bag (box) and inside it, roll the 120 roll film onto the spool of a 620 roll film.
Here, be careful not to loose the film of "120" (light blue in the picture).
On the contrary, on "620" (orange in the picture), roll up tight in first 2 or 3 turns and after that, roll it comparatively loose till the exposuring face of the film comes up.
When you roll a film on the spool about 5cm, strain the film of "120".
Repeat these procedures till you finish it.
The point is to stick the two together as shown in the right picture.

cameras which use 620 roll film

ANSCOFLEX2

KODAK MEDALIST