As was mentioned, adding sugar water to beer means you need to stir, which has the potential to aerate, which is bad. So be careful as you stir, do it gently, minimized any splashing.
Best way, imho, is to add the sugar water to the bucket first. Curl your siphon tubing so that its along the outer rim of the bottom of the bucket. as you transfer, it will naturally swirl the contents and mix everything up for you without any stirring.