To make cake mix from scratch, combine flour, sugar, salt and baking powder. Put 2 cups of the flour mixture in a food processor together with shortening. Process until mixture is very fine.
Each portion of homemade cake mix (about 4 cups) will make two 8" cakes or one 13x9" cake.
To make substitute for just one 1 boxed mix use:
2 1/2 cups (320 g) all-purpose flour
1 1/2 cup (300 g) sugar
4 tsp baking powder
1/4 cup (60 g) butter or vegetable shortening or margarine
1/3 tsp salt