The source text and the target or translation need to have the same number of sentences. The texts should be the same also in format, for example paragraphs presented in block style with a blank line between them.

If they are not the same, you must correct this. The assumption here is that it will be a one-to-one correspondence, source sentences match their translated sentences.

Paste the two texts into the respective text entry boxes and see if the number of sentences is the same.

If they are not the same, one way to resolve matching the texts by number of sentences is to segment each into sentences, examine differences and correct or punctuate as necessary. The texts must match sentence for sentence.

Given the same number of sentences for each text, paste both sentence-segmented texts into the text entry boxes and click on Convert.

Here are two tools to sentence-segment a text. On this site and at textanalysisonline.

Another possibility using the command line or terminal is discussed here.