Visual contact should be made through a barrier at first – for two adult cats a tall stair-gate can provide a good option, or for kittens keeping the kitten in their dog crate works well so long as they have somewhere to hide if they feel threatened.
Both cats should always have the choice to retreat or hide if they need to, and interaction should never be forced. Let the cats meet in their own time, and don’t pick them up or move them closer together.
Feeding both cats something tasty while they are in proximity can give a positive association with meeting, so long as the bowls are far enough apart for the cats not to feel uncomfortable.
If either cat appears scared or aggressive, gently remove the contact, continue scent-swapping, and try again the next day. Patience is key, and you should find that eventually tolerance will build up between the cats, until they feel confident enough to sniff each other through the barrier.