Explain to a second grader about how you subtract 2.34 from 20
be specific!

In order to subtract 2.34 from 20, the best method is using column subtraction.
To do this, we put the main number on top, and the number we are taking away underneath it.
When we set it out, we need to make sure that all the columns line up properly, the units in one line, tens in the other etc.
If taking decimals from a whole number, I would always recommend putting '0's after the decimal point for place value.
For the layout, see below:
   20.00
-    2.34

Now, as we cannot subtract 4 from 0, we must begin borrowing from the neighbours.
As 2 is the nearest number, we take one from there and move it next door
1¹0.00
Do the same again, 10 becomes 9
19.¹00
And one last time
19.9¹0

Now subtract 2.34 from 19.9¹0

   19.9¹0
-    2.34

10 - 4 = 6
9 - 3 = 6
9 - 2 = 7
1 - 0 = 1

17.66

See the attached image, I lined up the numbers as best I could
Hope it makes sense, once you get your head round it you'll be fine