Open the product details before the project file. Note the supported application and version, required plugins, included media and the elements the template expects you to replace.
Keep a clean source copy
Duplicate the downloaded package before editing. Work from the duplicate, keep linked assets together and record any font or plugin substitution made for the project.
Test one output early
Render a short representative section before completing every replacement. That catches missing media, unsupported effects and timing problems while the edit is still easy to adjust.