This is what I'm doing with William and Louis at the moment. William is a 5 year old castrated male and Louis is 8 weeks old and arrived yesterday.
William was already familiar with the smell of Louis as I'd been to see Louis several times so came home with my clothes smelling of him. Yesterday, I went into the back garden with William, with his collar on in case I needed to get hold of him quickly. My friend then carried Louis into the garden. It seemed OK for a few seconds, then William kept walking away from Louis and trying to ignore him, but growling each time Louis approached him.
It's pretty much the same now. They're not left together unsupervised and William chooses to ignore Louis as much as possible, but growls if Louis comes near him if I'm close by. If I'm a few feet away William has just started to touch noses with Louis, so it seems that the main problem is that he's jealous of Louis getting my attention.
I'm not telling William off and I'm trying to spend some "quality time" with him when Louis is asleep, but it's difficult when Louis is awake and following me around. Louis is a very confident little chap, which really helps. I reckon they'll be OK after a while, but it may take some time.