I recently had someone ask me to do a vlog on filler for their writing. While considering it I also thought about how long it had been since I had the opportunity to sit down and do a written blog. Since it had been quite a while, I figured I would set aside a few minutes and answer his question in writing. Whenever you write, there is always a lot of concern with the length of the story. Sometimes you want to make certain that the story is novel length (which varies, depending on who you are speaking to) and sometimes you want to make certain that you aren't overdoing it. It is the first of these categories that are concerned with the phenomena in writing known as "filler." In essence, filler is something that takes up space and helps serve as a kind of written bridge between one important event in a story and another. Filler makes certain that what you hoped would be a novel isn't just an article. I once heard someone say that one of the hardest things to do is nothing. Filler essentially does nothing for the characters or story, so it makes it one of the most difficult things to write.
So, if you are asking this particular amateur writer how to create filler for your story, here is my answer: don't. I understand how much it might seem that you need to come up with something to fill up space, but if you do, then it is possible that you haven't fully developed your story or characters yet. Each thing that happens in your story should be a link in the chain. Anything that isn't doesn't belong there. Whatever is being written should be something that helps the characters grow or the story move forward or give background to the situation. If it doesn't do one of those things, then you are wasting your time and your reader's time. Your characters and your story should be alive. Everything that is alive, whether they realize it or not, do things for a reason. They may not understand the reason, but it is there nonetheless. Your living story and living characters should be no different.
I understand completely how tempting it is to add extra stuff in to lengthen out a story. However, when it is all said and done, I would much rather have a well written short story than a poorly written novel. There's my two cents worth.