How can I model a diagonal gear?

Small

do you mean helical gear? you have 2 choose:
1- using geartrax software
2- using solidworks tool box
design library> tool box> Ansi metric> power transmission> gears> helical gear

  1. Step 1:

    left double click on "ANSI Metric (outline item)" in "SolidWorks Premium 2012 x64 Edition

  2. Step 2:

    left double click on "Power Transmission (outline item)" in "SolidWorks Premium 2012 x64 Edition

  3. Step 3:

    left double click on "Gears (list item)" in "SolidWorks Premium 2012 x64 Edition

  4. Step 4:

    right click on "Helical Gear (list item)" in "SolidWorks Premium 2012 x64 Edition

  5. Step 5:

    left click on "Context (popup menu)"