We start by writing first the length to be subtracted
from, and under that, we put the length that we are 
subtracting (cutting) from that length with a minus sign 
before it.  We line everything up nice and neat so that the 
numbers line up as well as the units ('ft' and 'in') and 
draw a line underneath, like this:

  25ft  8in 
- 18ft 10in
———————————

Now if we had larger numbers on top we could go ahead and
subtract.  But we can't. 25 is larger than 18, but 8 is NOT 
larger than 10.   So we must change it so that it is that way.
Here's what we do.  We: 

1.  borrow 1 ft from the 25ft making it 24ft. Scratch through
the 25ft and write 24ft above it

  24ft  
  25ft  8in 
- 18ft 10in
———————————

then

2.  Think of the 1 ft that we borrowed from the 25ft and remember that we have
    learned that there are 12 inches in 1 foot. So instead of the 1ft 
    that we borrowed, we must think of that 1ft as 12in instead.

3.  Then add that 12in that we are thinking of to the 8in, getting 12+8=20in.   
    Scratch through the 8in and write 20in above it.  So now what we write
    should look like this:

  24ft 20in 
  25ft  8in 
- 18ft 10in
———————————

4.  Now we have a larger number in the top row than in the bottom
row in BOTH columns, not just the first one. So we subtract the
numbers and bring down the units (ft and in).

  24ft 20in 
  25ft  8in 
- 18ft 10in
———————————
   6ft 10in

That's our answer:

Edwin