Usually easiest to get it most of the way in and have the outer CV in the hub with the bottom ball joint not in, then give give it a swing so the weight of the hub pushes on the drive shaft and pops the inner CV into place.
Failing that, get someone to hold the hub out of the way (without the outer CV in it) and line the driveshaft up with the inner CV so you're trying to push straight into the box, hold the inner CV in place and give it a push so the inner CV goes into itself and hits the CV into place (if you get what I mean).
Failing that, hold the shaft in line as above and bash the outer CV with a hammer until it goes in

Being careful not to damage the outer CV of course, otherwise it migth be hard to get the nut back on