I'm not good looking like Tsetsi, but I've used urine for sinus irrigation myself and will explain what I do.
First, I don't use a syringe. I use a Neti pot. Here is a site that my doctor gave me for how to use one: http://www.himalayaninstitute.org/Netipot/NetiPotGateway.aspx You can find them on Amazon, ebay, and many other places too. In starting out I irrigated three and four times a day but now that they are clear I do it almost once a day.
I make a saline solution of 1 teaspoon of sea salt to 2 cups of water at body temperature. Add to that a half teaspoon of soda and stir until dissolved. That's the basic solution. If you wish to use urine, take a half of the previous recipe and a cup of fresh urine and mix the two together, then irrigate with the Neti pot, about 3/4 cup through each nostril. The soda helps break through the mucus, and the salt helps to keep the water from burning.
I've been drinking my urine for well over two years and after drinking it for a week or two it became very light as opposed to the normal heavy odor of fresh urine. So, if you haven't been drinking your urine, you're going to have some burning with the urine. The saline solution does not burn and it flushes out the germs that are in the sinuses as well.
What I found better for me than irrigating with urine is to irrigate with tea tree oil. In the saline solution mentioned above, instead of adding urine, I add tea tree oil. Start with just a couple of drops and depending on how potent your oil is, you can go up to perhaps a dozen drops - that's where I'm at now. Tee tree oil is an antiseptic and it has an aroma that stays in the sinuses to keep them cleaned out for a few hours. Here's a site about the tee tree oil irrigation: http://www.frazzled-family-finding-change-growth-wellness.com/nasal-irrigatio...
I've been irrigating for a couple of months and my sinuses are the best they've ever been.
Best to you.