It's always good to vary your lesson, which is why a filler, in general, is not a bad idea. A filler is a small item that you add to the lesson or a little left over item from a previous lesson, that you add to the regular lesson, the main part of your lesson.

That's good because it adds variety to the lesson, but it's very often done the wrong way. If you start off the lesson with the main part of what you want to teach and then you add the filler, the students will realize that you are just adding in an extra thing. They are not going to be happy about it. If, on the other hand, you do it in a different way it'll work. Don't try to see if it will fit in. It might, it might not. The worst that happens is that it didn't fit in properly and that you have to work on the main part of the lesson. Maybe part of that will be a filler if you didn't schedule it right. If you have a difficult class that day then things take longer than is to be expected, then you could have created a problem. All of that is true.

However, if you start of with the filler and then you continue with the main part of the lesson, the students will accept the filler and the main part of the lesson. You will not have as much of a struggle.

Granted, it's very often difficult to allocate the right amount of time for different parts of your lesson. You might have a problem as a result of this method. However, as with everything else, there is a tradeoff. Your difficulties scheduling the main part of your lesson are minimal when contrasted with the difficulties the students might give you when they realize that you are just adding in an extra filler.

In addition, an experienced teacher can usually judge reasonably well and can tell how long each part of the lesson will take. They will be able to determine how long the filler will take and how long the main part of the lesson will take and decide of it can work out well. Even an experienced teacher makes mistakes, of course, but they tend to judge better than a new teacher and can hope that it will work out well.

At any rate there is no question that the right place to put in the filler is at the beginning rather than at the end of the lesson.

This flies in the face of everything that you were probably taught in your education courses. You were then taught to always have fillers ready and if there's time at the end of the lesson to put them in then. Well, I'm not too surprised. Most of the other things in this website also probably differe from your studies in your school of education. This difference should not be much of a surprise either.