Crankcase assembly

Hi can anyone explain how to line up the gear selectors with the correct gears when reassembling the crankcase halves. Any other advise would be greatfully recieved.

Put the shift drum in neutral & make sure no gears dogs are engaged [neutral] on the shafts. Line up the shift floating shift forks to the gears they engage. GET A MANUAL.

You need to assemble the gearbox in the upper half of the crankcase with the selector drum etc and locate the forks on their respective gear.
I.e. assemble the crankcase assembly upside down then flip it to complete the top end.
I suspect you currently have the gears sat in the lower half and are trying to locate the shift forks with them dangling down from the upper.