Type 'easy' or 'hard' to begin the quiz. Easy is a 2-degree grid, hard is a 1-degree grid. Then, try to name the largest UK 1K+ cities in each square IN ORDER. To name a city, you must first click on the grid square to make it have a GREEN border. Then, type in the city answer for that square followed by a '.' (a period).
The squares will start with '0/5' (or maybe fewer cities if less exist in that square). Click on a square and enter the largest city in that square not yet answered (so, the largest city if you have yet to answer a city in that square). You will then have '1/5' and need to click on that square to name the 2nd largest city, and so on until you complete all the cities up to 5 (or less if less exist). If you name an incorrect city in a square it becomes 'locked' and turns red and you can no longer accumulate cities in that square.
You receive 1 point for the largest city in a square, 2 for the 2nd largest, and so on.
At the end of the quiz the largest remaining city you haven't named will be shown in each square.
Quiz Takers 
the locations of the cities are very of, never in the center of the cities just vaguely in the right area.
Yeah the city centroids are based on the border of the whole city I believe.
These seem to be based on urban areas and some of the pins (Thisted for example) are way out in the countryside.
I think it’s just poor cordinates, A lot of the cities have pins right on a square border. Thisted is perfectly at 57N, Herning is perfectly at 9E. Hjørring and Brønderslev are perfectly at 10E, Helsingør is perfectly at 12º30’E, Vejle is perfectly at 9º30’E. etc.etc.
It’s centers of the boundaries I think based on the source (citypopulation), so yeah some aren’t going to appear right in the actual center of the populated area.
The pins on citypop are all accurate. I think you might be getting the municipal seat cordinates from the municipalities instead of the actual city. It would explain why larger cities like Frederikshavn and Helsingør are so far off while smaller cities and settlements in the municipality are more accurate. It’s actually quite bad, for example (on expert) it has Helsingør being in the square west of Hellebæk when it should be in the square east of it.
Interestingly the pins are accurate on the Denmark largest cities quiz.