The test strip has antibodies specific to the Covid-19 virus painted on it in a thin line.
Above this there is a window to the testing strip.
If your result is void, take another test.
One pinkish red line next to the control marker C, indicates a negative results.
Once you have dropped your sample into the well, a red line should appear next to the C relatively quickly.
Lloyds Pharmacy has one of the cheapest single test kits on the market at £1.