What system do the brickies put in place to make sure they get paid properly. As in installments like example laying foundations then asking for money for doing that. Thanks

On my last self-build I agreed a price per brick/block and paid them at the end of each week.  Anything extra was negotiated at the time.

Agreed day rates for work below ground then everything else is price work. So it’s a price per 1000 bricks and price per block laid. I pay extra for lintels and soldier courses or other detail and do that by metre. 
 

Invoice for work up to Friday afternoon gets paid the following Monday, bank transfer only. Any weekend work goes on the following week. 

Most trades would expect cash from a self build