Make the meeting away from your house, so you older dog won't feel the need to protect.
Get someone to hold the new dogs leash, and yourself hold the old dogs leash.
Walk past each other, at a far enough range so, that your dogs can't meet each other yet. If the dogs tense up, keep walking by at that same range.
Each time they look at the dog, relaxed, or just ignore each other, walk closer together.
Soon, they will be close enough to sniff each other... If the dogs tense, take it a step back, if not keep things going.
The dogs should sniff each other, then if they start playing around, you know its almost time. If they ignore each other, thats fine to.
Bring the old dog and new dog home, but keep them in different rooms, by putting up a doggy gate. They can see, and smell each other, but not start fighting, or playing.
Soon but the old and new dog on a leash and bring them into the same room. If they seem okay with each other, I'd say your good. But keep them away from each other at least another day.