Assuming your field1 and field2 are rows, you can create a row group. Generally groups rely on the dataset having the same value in a certain column.
e.g. If you have a "Country" column and some of the rows say "Canada" you can group rows by Country and all the "Canada" rows will be in the same group.
If you don't have any useful grouping column and you want to group all columns for the purpose of doing these totals, or divisions, you can use =1 as the grouping formula.
The easiest way to then do a total for the "sum[value] of field 1 / sum[value] of field 2" is to refer to the fields by their name (name of the cell/text box). Assuming you've named those cells "Field1total" and "Field2total"
So the formula looks somewhat like
From there you need to be careful about not dividing by zero - for example if the Field2total cell is zero you will have further problems.
What I've said here may or may not work depending on your exact layout but the "ReportItems!" reference is super useful I've found.