What is the most common way to change fields in academia? If you are a young scientist, how can you change fields? Is it better to make one big leap and apply directly to a new job or postdoc or trying to make small consecutive steps during your current job? What should be those small steps?

Just for explanation - in this case it is not like a change from literature to rocket science. The change would be in one broader field and some theoretical and statistical background would remain the same. What would be the best strategy, if you have some good papers in top journals in your field, but you do not have any papers from a field where you would like to move? And there is no such laboratory in your country, so there is no personal contact.
If this is a move within the same broad area (or a closely related field) your best bet is to get or keep a job in your own field, and then begin finding research that transects both of your fields. For example, if you're in political science currently and want to shift to, say, geography, then begin by publishing research on geospatial politics until you have enough publications in geography journals to get a geography job. Often, schools will have interests in more than one field, and so a background in more than one can be a benefit when looking to advance.

What would be best is if you are in a school that has both fields of research - in which case collaborations would be the easiest way to go. I'm currently in the process of doing something similar, but I have the benefit of being in a very broad school that has both people in my own field, and people in the field I am shifting to (slowly, but surely!).