Commonly used commands:
Polygroup mode, when there is more than 1 subdivisions created from shapes:
PolyEd: choose the polygon and edit
Tri model for triangle polygon composing, instead of square
GrpGen in Attribute, control level of selection in tolerance level
After selected, go to PolyEd to modify the shape
MatEd (Material edit) tool to select by painting the area:
Or change all in group in selection mode
Add material and click Assign Active Material