yeah this is difficult to do. i found even more so when i had a centre brace running right across the top of where my baffles are! it is a messy job with silicon, if you get it wrong and have to move it silicon goes everywhere. best thing to do is mark on the outside of the tank, 3 vertical lines on each side of the tank where you want the baffles to go. the first baffle is easiest of course as its accessible from both sides. when you silicon the next in, run a bead of silicon about 2mm outside the 'marked' vivid line. when you put the baffle in, push it evenly so it spreads the silicon across the entire edge of the baffle to your original marked line. make sure the baffles are a couple of mm smaller (on each side) than the width of the tank (in other words dont make it a tight fit or you will find 1. it is difficult to get the baffle in and 2. you'll have a bad join, its the silicon thats doing the sealing) i also gave about 1.5" gap between baffles, 1" is just that much more difficult to work with - and also if you drop anything between the baffles and need to get it out!