When people usually refer to sound effects, especially in terms of post-production for film and video projects, they usually mean audio samples that were not recorded with the video. This non-diegetic sound is oftentimes either recorded individually through folly methods, where different actions are made to mimic a specific sound, or through purchasing prerecorded audio samples that are employed. From here they are collected and imported into your video editing project to be worked into the general soundtrack so that they sound as though they could have been recorded as part of the scene. Here are a few tips for how to import these sound effects into your video editing project and use them effectively, whether or not you have created your own sound effects or download sound effects.
The first thing you are going to want to note is that you do not want most sound effects to stand out too much as this will highlight the fact that it was not recorded directly. Most people would recognize its artificiality if they heard the sound effect clearly, so you want to blend it. You will often do this by lowering its overall volume, and possibly use a combination of sound effects or a mix with the natural sound. What you want to do is to fit it into what is a general collage of sound that makes up the overall soundtrack. If the sound effect that you have brought into your video editing project does stand out too much you may want to begin applying cross fades to the beginning and end of it and try to apply audio filters if necessary.
With audio clips of sound effects, usually the shorter the better. For example, crashes are often used for sound effects where nobody wanted to actually break an object for each take on set. You can then employ it into your video editing project at the right moment, lower the volume, blend it into the music and ambient sound, and then it will fit. If it is too drawn out, as often happens when sound effects are recorded on their own, then it will not feel natural. Unfortunately, most sound effects cannot just be trimmed as they have an arc from beginning to end. The best option is to actually try several different clips of the same sound type to find one that will eventually fit.
Accuracy in Timing
The sound effect, as with all sounds, is the result of a conflict or production taking place. This means that the sound must be inspired by something that occurs in the story space, such as a person dropping a glass with a resulting crash. If you want to maintain the continuity of this, which is the main priority in most uses of the sound effect, you want to make sure that you match the timing of the sound effect with the action that caused it. This action does not even have to be on screen, but it does have to make sense for the physical space and interaction of the scene that was just taking place.
Add Sound Effects Early
You are going to want to include the sound effects before the sound mixer provides the first mix. It is especially important to add sound effects before music as it is going to be part of the general sound of the story space, yet it is understood on some level that the music is not. This means that you have to blend it as best as possible with the regular audio physics of the scene before adding thematic elements like music.