Whereby you give her a ring that symbolises posession.
The origins of giving an engagement ring are lost in the dim and distant past, back when cavemen supposedly tied grass ropes around the ankle of the woman they claimed, the better to keep them near the cooking fire.
We are far more civilised now.
Bonding jewelery until classical antiquity times was more usually actual chains or bracelets, symbolising the possession of the woman by the man. The earliest acknowledged engagement rings were found in Ancient Egypt, where placing a ring on the fourth finger left hand was considered to be banding the love vein. The Romans, disdaining the traditional materials of hemp, leather, bone, or ivory as barbaric trinkets, made betrothal rings of iron, often decorated with a key. This ring served as a slave band, showing that the woman was a chattel and so not available without negotiation.
Like apartheid, tulipmania and drug for weapons exchanges, we can blame the whole modern engagement ring phenomenon squarely on the Dutch. Hence the old phrase "getting into Dutch" when you fail to buy an expensive enough ring. After a saturation advertising campaign by De Beers, 2 months salary has come to be the norm to spend. Many women angle for three months salary - if you have one of those, release her to the wilds and find a more realistically priced woman.
WHAT! No diamond!?BASTARD!!!!
For the Ladies
Look - he bought you a ring. So what if it is not the style you wanted. He'll not remember what it looks like - take it back and exchange it for the one you acutally want.
For the Men
Not to put more pressure on you here, but your choice of ring is going to be judged by her family, friends, work collegues and nosy passers by. Accept that you will fuck up the selection if you go on your own. Either take her with you - boring, or make sure she knows where the receipt is.