Once you have completed the project schedule and network in the project editor, the next step is to add buffers, via the "CCPM property dialog" box or "insert buffer box".
If you project is planned "Just in Time" (from right to left), inserting the project buffer will push the critical chain to the left. The effect of this step is that due to the structure of the plan the size of the critical chain may change.
This may also impact the size and position of other feeding chains and buffers (feeding buffers and/or milestone buffers).
When this happens a message box is populated, with which you can control this buffer process as a two step process.
See this video in which buffers are added in a sequence of 3 steps, triggered by the automatic detection of changes in a Critical Chain as a result of adding the first buffer (project buffer).
See also the attachment to this article with examples and further explanation.