Most times, especially early on, you are going to lose some wine and it will need to be replaced. So you can used distilled water to prime your siphon line.
OR if you are testing before racking (Should test before not after racking anyway) Use the wine from your Hydrometer test tube to prime the siphon line.
This assumes you don't have an auto siphon.
In most cases without one there is going to be a little spill - I just learned to manage things carefully using my clip shutoff and holding both ends of the siphon line at the same height. Using distilled water it takes a bout 2 to 2/12 ozs of water to fill my siphon line. That amount is not going to dilute my wine enough to worry. OR as mentioned using the wine from my testing avoids that completely. I draw it with the wine thief and use a clean 1 cup measuring cup to pour the wine or water into my siphon line.