You generally use a feature which you can see to run along the taxi line which keeps you on it when you first start taxing. Then its all done by experence and it looking right.
Eg on the Jetstream 31/32 (I know its only icle) you run the bolt on the windscreen wiper pivot along the line and you will be bang on the taxi line. In fact its the most usefull thing the wipers do on that aircraft because they sure as hell don't remove any water from the screen.