Had cause yesterday to create some subtitles for a video. Not done this beforehand so was more than happy to come across Subtitle Editor which did exactly what I wanted with very little knowledge/ experience on my part. Without needing to download anything (other than the resultant .srt file) or create any account. Perfect!
Consequently I would recommend it to any similarly amateur people like myself.

Subtitle Editor - Permanently Free, Open-source, Fully Web-based SRT/VTT Editing Tool
Edit, create, and align SRT/VTT subtitle and captions files easily with this free, open-source, web-based editor. Features video preview and waveform visualization. No signup required.

