So first off, you have to have the gray card in the frame on at least one exposure. Say you took 5 images with the same lighting, same exposure... one of those 5 needs to have a grey card (or similar neutral density material) in the exposure. It's important that that gray card is what you make your test strip of, and that that is the one you have in the dark room to compare to.
Say your test strip has 10 sections, 1/2 second apart, starting at 2 seconds, and going to 7 seconds. Develop it, fix it, rinse it etc, just like you would a print. (though to be honest, you don't need to go the full fix/rinse and there's no point drying it, because you're only going to look at it once then toss it.)
Take the test strip and your grey card outside the dark room into normal room light, or sunlight. Hold the strip next to your grey card and figure out which strip on it most closely matches the grey card. Which ever that strip is, that's your starting exposure for those 5 frames, on this enlarger, on this paper, with this batch of chemistry. That's a lot of variables you're testing for... here's how they can impact you:
- Different enlarger? Different bulb strength, or clarity of optics makes for a different exposure. Heck the dust layer on the optics can impact it too.
- Different paper? Different papers react differently, require different amounts of exposure.
- Different chemistry? the last print through the trays of old chemistry about to be replaced and the first print through the fresh trays... WAY different. Especially if they are from different batches or mixed by different people.
- Different roll of film? even if they were the same exposure and lighting, if they weren't processed at the same time, or even if they were if they were hand processed, the negatives can come out slightly different density.
All those things (and more) are why you make the test strip in the first place.
Note that you may need to eyeball it between two frames... if 5s is too light but 5.5s is too dark, then maybe 5.25s is what you want to start with. Once you're more comfortable with the process, you can probably make fewer stripes on the strip and eyeball between them more.
Important note, this is your initial exposure time. Do a work print with that time and look at how it comes out, make your artistic decisions if you need more time or less, need to dodge/burn to compensate for exposure issues in the film, etc. Work your way through several work prints if you need to before doing your final full size print.
Now, I implied above that the grey card was only good for those 5 frames. But the reality is that as long as you are consistent in your exposure on the roll of film, you can reuse those settings from shoot to shoot too. If the first 5 frames of one shoot and the next 10 frames of another shoot are equally exposed, then you can re-use the time you got from the first test strip across both sets of images. One way to compare the exposures before you get into the dark room is to put two negatives next to each other on a light box and look at the grey cards in them... do they look to be the same brightness? (tip: lay a piece of black card stock with two small holes punched in it over the negatives and line the grey cards up with the holes.)