Make a new project, then import the video that requires the blurring effect.
Drag the footage from the Import or Project window into the first video track of the editing timeline.
Duplicate the footage by copying and pasting it into the second video track. Make sure that the two duplicate footages are placed right on top of each other on the two tracks. Use the second video track for the creation of the blur effect.
Click the "Effects" button or any similar menu in your program, then select the Blur effect. Most programs have different types of blur effects. Usually, the Gaussian Blur is used for this process.
Set the amount of blur you want to include in the video. This is usually in numerical form, or there can be a slider option that you can drag from left to right. At this stage of the process, the entire video is blurred.
Add a Crop filter, which is typically found under the same menu as the Blur effect.
Configure the crop parameters depending on where exactly you want the blur to be placed on the video. These parameters include the left, right, top and bottom of the part where the blur should appear. If the part being blurred slightly moves during playback, make the blurred area big enough to accommodate its movements. If the element being blurred moves around different parts of the screen, use keyframes to animate the blur effect you made. Most amateur editing programs, however, don't have this feature, so you have to find a professional editing program, such as Adobe Premiere Pro or Final Cut Pro.
Mark the keyframes for the video's beginning and ending movements by matching these keyframes with your required blur effect. Keyframes allow you to change a specific property or effect on a video during playback. The keyframes typically appear by expanding the editing timeline. This may also be accessed through the "Effects Control" option under the program's "Effects" or "Video" menu.
Set the desired value for each keyframe so you can manipulate its movement according to the particular element on the video that should be covered by the effect. Also, set the duration on how fast or slow the blur effect's movement should be during the video's playback.
Render the video to finalize the effect.