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First, you are going to want to indentify the words that are to be included in your subtitles and accurate transcriptions need to be made. You will want to then go and set Markers in the Timeline where the blasts of language are occurring on the audio track where you are going to place an associated subtitle. These are where you will know to place the subtitle blocks that you create.
Go to the Browser and select the Effects tab. Go down to the Video Generators folder, and within that you will

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want to select
Text. There are going to be a few text options and you are going to again hit the one that is simply titled Text. This will bring the Text generator into the Final Cut Pro Viewer, and when you select the Controls tab you will be able to add the text and make alterations to its font, size, tracking, behavior, and other text qualities. It is here that you will basically create your subtitles in Final Cut Pro, and once you have you will select the Video tab again.

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You will click on the video as it appears in the Viewer and drag it down into the
Final Cut Pro Timeline where you want it to be, and this should be indicated by the Marker you placed earlier. You will then want to shorten the clip so that it matches what would be a reasonable read time and so that there is room for the following subtitles to fit without overlapping.
Once the selected subtitle is in your Final Cut Pro Timeline where it matches up with the audio you are translating

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into a visual form you are gonig to want to reposition it as it will be displayed right through the middle of the frame. Select the clip and then go over to the Çanvas. Make sure that the Image + Wireframe has been selected so you can move the position of the text around the frame. Move it to the bottom third of the screen so that it is below the primary action of the frame.