Here ‘practise’ means ‘carry out in action’. The moral is: Behave in the same way as you advice others to behave. Do not recommend early rising if you yourself lie in bed till noon.
‘Don’t keep on sniffing, Johnny,’ said his sister. ‘Blow you nose.’
‘You’re a one to talk!’ Exclaimed Johnny. ‘You sniff more than I do. Practise what you preach!’