Dear Desperate, let's see if we can keep this simple (and understandable):

1) Video on timeline
2) In the Media Generators tab, click on Solid Color and drag Black to a track above your video. Close the box that opens up.
3) Position cursor the thin line some people call the scrubber. You should see all black in the preview. Don't worry, you'll size it later.
4) In the Media Generators tab, click on (Legacy) Text and drag Default Text
5) In the text box that opens up, In the Edit tab, choose the font and size, type the text you want. In the Placement tab, position it how you want.
6) Back to the black generated media, on the timeline click on the little Event Pan/Crop icon (the bluish looking thing) to bring up the properties window.
7) Click on the plus sign next to Source, change Stretch to fill frame to No.
8) On the left make sure Move Freely (X or Y) is chosen (lines with 4 arrows), you may have to deselect the other two icons above it.
9) In its preview area, drag the corners such that it is just larger than the text.

Hopefully by now you can see your white text with a black background and looks sort of what subtitles should look like. If so, proceed to the next step.

Drag the Text and color events to the the position you want, click drag the leading and trailing edges to get the duration you want.

Then select the text event, shift click the color event, Copy. Move cursor to trailing edge of the events, Paste. Select Create a new copy (which should be the default.

You should now have a second set of events that are the same as the first.

1) Click the film strip icon to edit the text with the next set of words.
2) Click on the color event to adjust the cropping to match text size.

Repeat until done. Render your video using your desired template.

I'm sure there are many other ways to do it and others may suggest simpler ways, but this should get you going.

You could, if you want, crop the color event to one size and leave it static for all text events. By doing this, you've created a lower third of sorts.

Post back if you need more help.