Check with your vet first before giving any kind of drops. Here's a homemade solution frequently used on my feline herpes group, but I've never tried it myself.
A general recipe is 1/2 teaspoon salt per cup of water with a pinch of baking
soda (the baking soda is the "buffer"). Sterilize the water used, if not
purchased sterile, by boiling. Distilled water may be best for this purpose.
Noniodized salt is usually recommended. Most table salt is iodized but
noniodized is right next door on the grocery shelves.
I keep my saline in a sterilized glass jar and use a 1 cc feeding syringe to
administer the drop. You can sterilize a clean glass jar by filling it with
water just off the boil and allowing the bottle to stand until cool, then empty.
Or you can fill with water and microwave a minute or two just until the water
beings to boil. I draw up 0.5 cc and hold the syringe (no needle obviously
:-) in the palm of my hand to warm the solution. Sterilize the syringe by
standing in a little glass and adding water just off the boil, let cool.
Of course you don't have to make a full cup, you can halve the recipe. It's easy enough to make up fresh frequently, there's no need to finish the bottle before making fresh.
After I've warmed the syringe and solution, I tilt the cat's head back and place a small drop on the fleshy part of the nose between the nostrils. A
little saline can then trickle down each nostril. And if you miss or they shake
their head, just try again later - it's only water.