Open the 'Customize' dialog, select the 'Commands' tab, and in the 'Categories' list, select 'New Item'. Now from the 'Commands' list, drag the 'New Gallery' item to which ever Ribbon tab that you want it. Once it's placed, while still in 'Customize' simple find and drag your desired icons into the new 'Gallery' item. Once you're done, save your 'Role' and you'll be good to go.
It is possible to use a file based approach. And it is possible to use a tbr file so that the "toolbar" can be used in Legacy(Classic) mode or Ribbon mode.
To get the small icons wihtout text the tbr file will need an additinal line after each BUTTON line containing the following:
The tbr file will simply ignore the ribbon directives if NX is running using Legacy UI.
To control the suggested presentation of the "group" (from tbr file) the following two lines can be added (once) after the VERSION direction and before the first BUTTON directive:
COLUMN_IN_RIBBON 4 COLUMN_IN_POPUP 4
This allows one tbr file to be used for both user interface modes and to easily deploy the "toolbar/ribbon group" from a centralized shared location.
Example TBR file:
TITLE Toolbar Title VERSION 170 COLUMN_IN_RIBBON 4 COLUMN_IN_POPUP 4 BUTTON UG_MODELING_HOLE_FEATURE RIBBON_STYLE SMALL_IMAGE BUTTON UG_ASSY_SEL_BASE_COMPONENT RIBBON_STYLE SMALL_IMAGE BUTTON UG_MODELING_CYLINDER_FEATURE RIBBON_STYLE SMALL_IMAGE BUTTON UG_MODELING_CHAMFER_FEATURE