Main thing is prep. Get the tank absolutely clean and dry. Also, new silicone will not adhere to old silicone, so remove whatever you can reach that's inside the tank.
Then, run a continuous fillet down all seams. Good idea to do the upper rim, too, to prevent any capillary action.
What used to work well for me was to use painter's tape to mask off either side of each seam after I prepped the tank. Run a bead of silicone down each seam, then use your finger or a tool to make a small fillet that spans the joint. Peel off the tap, and viola! - nice, clean seams.
Take your time, and good luck!