A table that looks "properly set" is doing something practical: everyone gets the same amount of room, nothing is reached across, and every piece sits where a hand expects it. Once you know the handful of rules behind it, you can set a table in under a minute with whatever plates and glasses you already own. No matching sets needed.
Almost all of it comes down to a few distances. The plate sits close to the table edge, cutlery lines up beside it, and each person gets a strip of table about as wide as an outstretched pair of forearms. Try it below: drag each piece to where it belongs.
Put the plate down first, about 2–3 cm in from the table edge. Closer and it tips toward you when you lean in; further and every forkful is a reach. If you have a bigger table and a smaller plate, line the plate up with the others rather than centring it in the space.
Everything else is placed relative to the plate. That's why a good setting looks tidy even with mismatched crockery: the lines of the cutlery and glasses match from seat to seat, even when the plates don't.
Forks go to the left of the plate. Knives and spoons go to the right, with the knife's blade turned toward the plate. Set them from the outside in, in the order they're used, so the first course's pieces are furthest out. Keep them parallel, with their handle ends in a straight line with the bottom of the plate and about 1–1.5 cm of space between each piece.
Dessert cutlery can wait in the cupboard until it's needed, or lie across the top of the plate: spoon handle pointing left, fork handle pointing right, fork below the spoon. For an ordinary supper, one fork, one knife and perhaps a spoon is plenty.
The water glass goes directly above the knife. If there are wine glasses, they step out diagonally to the right, so nothing needs reaching across the plate. Leave a centimetre or two between glass and plate edge so elbows don't knock them. The napkin goes to the left of the forks, or sits on the plate. Both are placed to be picked up from the seat with the dominant hand, which is why the glass is on the right.
That's the whole rule set. Candles, a runner or a small jug of flowers are a bonus, but give them a spot in the middle that leaves each place its full width. If a centrepiece blocks two people from seeing each other, it's too tall or too wide for the table.
A common rule of thumb is about 60 cm (roughly 24 inches) from the centre of one plate to the centre of the next. That's what the planner above uses. You can squeeze to around 55 cm for a short dinner among friends and nobody will mind, but below that elbows start to collide, especially when people are cutting food.
Seats at the ends of a table are usually more generous, because they don't have a neighbour on one side. Don't forget the chair: it helps to leave around 90 cm of clear floor behind each chair so people can sit down and get up without squeezing past. If your room can't give that, pushing one long side against a wall and seating people on three sides is a perfectly good solution.
Measure your table before you plan a dinner for a crowd: its length, and its width if you want people at both ends. Then count places with the 60 cm rule and you'll know how many chairs to borrow before anyone arrives.