Going from memory there are three or four bolts that hold the shift lever cover assembly to the top of the transaxel. Take this cover off and then carefully pry the shift rails so the slots all line up. This will put tranny in neutral. The end of the shifter is probably worn enough that it can slip from on shift fork to other with out first one being fully in neutral,
The detent springs were also prone to failing on these trannies. You have to split the transaxle to replace them though