1. Cold set frame to 120mm
2. Remove outer chainring and move to inner chainring to outer postion if needed to achieve desired chainline. Use short BMX chainring bolts.
3. Attach BMX freewheel to rear wheel.
4. Remove 6 mm of spacer from rear axle. Remove from eitehr side to achieve proper chainline but the total removed needs to be 6mm
5. Re-dish rear wheel
6. Cut chain to length

Parts list:
BMX freewheel: $15
BMC chainring bolts: $8

I usually go back and forth with the spacing. Initialy I'll take all 6mm from the gear side of the rear wheel and see how the chainline matches up. You know the rear is 120 so that's 60mm from centerline perside. Measure the distance from the inside of the right dropout to the center of the freehweel teeth (15mm as an example) and that's you 'centerline'.

60-15 = 45

Now go to the front and measure from the center of the downtube or seat tube to the center of the front ring. This number should match rear number or 45 (for this example). Sometimes it's tricky to eye up the center of the tube so measure its diamter and divide by 2....30/2= 15. Now measure from the side of tube to the center of the chainring and add the number together.