Experience comes with time..
Don't slip until you're entirely sure he seen it and is straining on the slip..
I personally give them sitters, that i walk right out to..
Once they can bang them on the seat or are going in the right direction ,then I try and give them longer slips, as time goes on..
Once they click they will run ten meters or one hundred..
Keep going.. Loads of cover and tall grass..
Its harder for them to spot them but once they click they will run the beam and trust there is something worth while at the end of it..