There are several ways to transition into a flashback. It can either be a smooth transition that is eased into and out of carefully or it can be a jarring transition – one where the character and therefore the reader are both intentionally jolted into the transition – either through a physical or emotional trigger.
Flashbacks can occur mid chapter, or sometimes, to avoid any clunkiness in the transition, they can be included at the beginning of a chapter. One of the most important points about a transition is that the ensuing passage or flashback must contrast with the preceding and following passages. This is particularly important as it clearly signals to the reader that there has been a shift in time. The transition must be purposeful and consistent both into and out of the flashback.
Any shift in time will affect the action both before and afterwards – particularly so as the reader's perspective will be altered and they will potentially go back to readdress events they have already processed. Remember when you continue to write that you must keep the action moving forward and that the reader will be looking on with a deeper knowledge or understanding of your main character and therefore a new perspective.
The most important thing to remember is to only use a flashback when there really is no other way to get the information across that you absolutely need the reader to know. The flashback is a very handy literary device; however, it can be a hindrance if used too often or incorrectly.