In the UK, ballots are still done with pen and paper. You put a cross in one of the labelled squares and fold the paper, then drop it into the ballot box.

Also, while the guidelines say it has to be a cross, it could be any clear mark (though best not to risk it), as soneone drew a rude symbol in a square and it was counted as a vote! (https://www.bbc.co.uk/news/magazine-32693485)