Literally that - if you pick the right
two fingers - such as your little fingers to start with, then you should
be able to get both inside your foreskin. Note that you do this in the sequence - get one little finger in, use the thumb of the same hand on the outside pressed against the finger inside to get a grip on the foreskin so you can push the second one in alongside it.
Note that while the diagram
shows the fingers held apart, until you really achieve some stretching, you will generally have the two fingers against each other, which is actually easier, because the stretching is done by "levering" them against each other and this will not tire your arms. Note also that since the space inside the foreskin is much wider than the tight part, the fact that the two fingers are "hooked" inside the foreskin, one either side of the glans, prevents the foreskin from slipping off.
Now, if indeed it is so
tight that you cannot fit two little
fingers inside, then concentrate on getting your thumb in there, and practice getting as much of it as possible inside the foreskin, sitting beside the glans. Keep your fingernails trimmed and smoothed. Although it may make it more difficult to get a grip in order to insert the finger or fingers inside, a proper lubricant (never soap, shampoo or detergent, or scented, or motor oil) will make it more comfortable and more effective.
And incidentally, unless she has long fingernails, I am sure your girlfriend has smaller fingers!
So - the way to do it is by using fingers - when
you have stretched it with your fingers, then
you will be able to retract without the "tight band".