This is a common theme across the books and podcasts, though some advocate taking it to extremes. Essentially it means that you set a standing order to pay into your stocks and shares ISA as soon as you get paid, or whatever your investment vehicle of choice is. It then becomes just another bill, excluded from the mental budgeting we all do was we bridge the seemingly endless void between paydays. It’s true to say, if you save/invest what you have left at the end of the month, it will pretty much be zero.
I’ve been doing this for a few months now, and there has been a couple of months when it’s been difficult due to car bills etc. I think you need to be realistic with the amount. 10% of your take home pay is a pretty good start, and would compound to a useful amount in the long term.