They are never perfect by any means, but they are good enough. A few tricks / the steps I have found by doing several of these.
Things you will (probably) need: welder, a set of dollies and body hammers, a good (90* air grinder and roloc wheels is my preferred tool of destruction for grinding and sanding everything smooth) grinder and abrasives, several punches/bolts/anything to align the pieces to the existing top, a drill, a correctly sized drill bit for your rivets, several drywall/wood screws that fit in the rivet hole, a flat bench or a relatively flat floor (very much preferred, but not 100% necessary), some clamps, some sawhorses/table, a friend, and time.
1. Get the top up on a body dolly/table/anything to get it off the ground.
2. Roughly lay out all of the pieces, get organized, check the rough fitment of the rounded corners and visor pieces. (These seem to be hand bent, and are usually close, but not perfect, and are usually different radii than the factory corners.)
3. I like to start at the back, and work my way forward, but whatever you like. I layout and pin and clamp each piece on the existing hardtop drip rain, with the bottom (flat mounting face) of the new rail facing the current hardtop rain gutter bottom (flat mounting face). I like at LEAST 2 pins per section, but more is usually better. I leave the visor section off till later.
You will probably need to make adjustments to the corners to get a flat, even fit up. I usually have to rebend the folds on the pieces to make them deeper, and usually have to take some material away to get them to fit. Take your time and get the fit flat and tight, once you start tacking and then flip it over, it will be very apparent if you don't get everything flush. A good set of body hammers and dollies really helps here. Don't be afraid to make slices to reweld if something really doesn't fit well. I usually have to do this to make the corner pieces match the width of the straight gutter sections.
4. Once the fit is acceptable, and you have ensured that each piece is close to flush and flat, and there are no major warps or twists across the entire gutter, start lightly tacking each piece together. I usually only do 2 tacks per piece and will finish weld the opposite face.
5. I next trim and tack the visor sections in, making sure it fits well and matches the curvature of the hardtop.
6. Once everything is tacked and looks to be correct, I like to pull everything (an extra set of hands really helps here) and set it on the ground / flat table to ensure that the entire gutter is flush when welded.
7. I slowly weld the sections together and grind. once welded, double-check fitment against the original hard top, then grind. I like a 90* die grinder with a 1.5" 36 grit roloc disk to remove a majority of the weld, basically flush with base metal, then a 3" 36 grit to blend the weld with the base metal. Don't over-grind, and make sure the pieces are flat and even before welding. If they end up warping, straighten them as much as possible before grinding to prevent taking too much material.
8. Once everything is ground and ready to be installed, I start popping all the rivets on the hardtop, by grinding the head and punching out.. If you have any leftover seam sealer on the old gutter, now is the time to remove it. A good fitting punch is your friend to pop the rivets out. Peel the old gutter from the fiberglass.
8.5 (optional) I've never done this, but now would be the time to paint the gutter if you are not planning on painting the entire top. I've always had the top completely restored, so everything is painted after the seam sealer is applied.
9. The fiberglass will probably warp/move, this is expected. If there is a lot of movement from the fiberglass, I get several 2X4/6's and lay the new gutter on them, flat face down (like it would be on the vehicle). With a set of extra hands (or by yourself, I've had to do it both ways), place the fiberglass on the new gutter and center it. If there is a lot of gap because the fiberglass warped, press the fiberglass into place with a friend. While the top is pressed in place to fit properly, drill a few holes into the new gutter, based on the existing fiberglass holes, and secure the fiberglass with the wood screws. Even if the fiberglass fits well, this helps a ton if you are doing this by yourself.
10. VERY Carefully drill all the new holes in the gutter. You can very easily mess up the straight rivet lines if you aren't careful, (I have done this a few times), and will need to reweld and drill the holes. The drill will walk very easily in the fiberglass, so a sharp bit is very important to keep everything straight and happy.
11. If you had to screw the fiberglass down, try and tightly clamp the gutter to the top to ensure it doesn't move when you flip it. Pull the wood if you had it attached. Flip the top over so it is interior side up (opposite of the vehicle), and place it on your bench/sawhorses/whatever.
12. Riveting is the difficult part of this to keep everything looking OEM, but you can do whatever. I believe CCOT supplies the correct length rivets with this kit. I use a homemade punch set that replicates the OEM style rivet head and is paired with a stand that accepts the existing rivet head. (I'll add a drawing of the stuff I use). Rivet the top down. I place the existing rivet head on the fiberglass (the wide head), and make the punched head face the flat mounting surface like OEM.
13. Flip the top and gutter back over, so the top is back as it would normally be on the vehicle. MAKE SURE IT IS VERY LEVEL I use 3M Self Leveling seam sealer (2 tubes) to seal the gutter to the hardtop, but you can use whatever you can find. This stuff sets SUPER FAST, so work quickly. Fill the gutter with the correct-looking amount of seam sealer, and don't forget the top rivets that hold the inner metal piece on.
14. After this, I have always prepped the top for paint, but you do you.
Hope this helps some. It's a pretty easy replacement.