I have never done this before, but this is how I would attempt it.
First step is to remove the trim from the top and bottom of the tank. You can find instructions on how to do that elsewhere in the DIY forum.
Next you need to remove ALL of the silicone from EVERY joint. This is the most important thing to do, because new silicone does not adhere very well to old silicone.
Once all of the silicone and silicone residue is gone, place the bottom piece of glass inside the bottom trim piece. Now lay down a generous bead of silicone around the perimeter of the top of the bottom piece of glass.
Now take the front and back pieces of glass and lay a generous bead of silicone on the inside face of the glass ONLY where the side pieces will attach.
Depending on the size of the tank, you may need to enlist the help of a buddy for the next step. Place the front and back pieces where they go on top of the bottom. Don't worry about the silicone that will squeeze out.
Now lay the top piece of trim on the top of the sides. This will help hold everything in place.
Now working from the INSIDE of the tank, take one of the side pieces and tilt it toward the inside of the tank. Then slide the bottom edge of the side into place while making sure that the piece remains tilted to the inside.
Now pivot the side into place. This will cause most of the silicone to be squeezed to the outside of the tank. Repeat for the last piece.
Now you have a big sticky, smelly mess of silicone squeezed out of all of the joints. Now you will need 4 pieces of wood that are as long as the tank is tall and 4 bar clamps that will fit over the sides of the tank.
Place a piece of wood at the edge of the front and back of one side of the tank and place a bar clamp at the top and bottom of the wood.
Clamp it down LIGHTLY and make sure that the side piece is in the right place, square and plumb.
Once you have the side aligned tighten the clamp more, but don't overtighten. You just want an nice firm joint and you don't wnat to apply too much force. If you have gotten this far, I'm sure you can figure out the right tightness. Repeat for the other side.
Now that you have it glued, clamped, squared, and plumbed, here comes the fun part. You will definatley want to wear rubber gloves for this part and I would reccommend wearing them for the entire project.
Clean the silicone off of the OUTSIDE of the tank. You don't have to get it all now, but try to get most of it.
Next you will want to smooth the silicone around the INSIDE joints creating a nice clean radius on all of the seams. You can add more silicone if need be.
Once you are happy with it, let it dry and fully cure.
Once it is cured you can remove the clamps and use a razor blade to remove any silicone from the OUTSIDE that you missed while it was wet.
Now give the whole tank a good cleaning and you should be ready to add water.
Fill it all the way and check for leaks.
Once you are satisfied that it doesn't leak, cycle it and use it as normal.
Like I said, I haven't done any of this, but this is how I would attempt it. I hope this helps more than it hurts.
