It was simply a matter of leaving a flap that would fit behind the line, top and bottom. I had to have two markers, one which served for the one and two place numbers, and another to change to for the three place numbers. With this permanent set of numbers visible to all, and laid out in a line, we had a very useful tool for a lot of different work. Together we did skip counting, adding onto and taking away from large numbers, multiplication, comparisons of value, place value work, and the concept of before, after and between. By thinking of the line extending around the room several times we could even get a sense of how really big a thousand would be.
Some teachers prefer to form the same kind of line as the days go by.
They start the rolled up line at 0 and print on a new number each day, extending the roll as they go. The youngest children then have the advantage of seeing the numbers printed out, but the older children loose the advantage of having the full line to refer to during the year. I haven't done this, or seen it used, but I believe that teachers who aren't able to have a number line stuck up around the room could make a large number chart to serve the same purpose. It would have to go from 0 to beyond the normal school year, and be spaced out to allow up to three figures. Some simple marker could be used to move along the chart if the numbers were already printed.