This is a mix of plain flour and cornflour. The cornflour should be around 12% of the cake flour.
e.g. if a recipe needs 375g cake flour:
0.12 x 375 = 45g
45g cornflour + 330g plain flour.
Remember to sieve well!
I don’t tend to buy this anymore, just make my own from plain flour and baking powder. This ace post explains how much baking powder you should use in more detail, but essentially it’s 5% of your flour.
e.g. if a recipe needs 250g self-raising flour
0.05 x 250 = 12.5g
12.5g baking powder + 237.5g plain flour
Again, sieve well.
Converting from cups to grams
Not a fun process, and every site seems to say something slightly different. Here are the ones I’ve worked out for myself:
1 cup plain flour = 125g