The BFI is a Big Intake for getting air to the engine. It's pretty simple to do, no special tools needed. Different people have made them with different materials, I suggest start with an easy one, then you can make it better later once you know what you are doing.
Materials needed, 20mm foam, preferably the type with a foil coating on one side, it comes in a 1 metre roll, you only need 1/3 metre or less. I also used some coreflute plastic, the stuff advertising signs are often made from.
First, remove the drivers side headlight. See here
Remove the inlet duct which is held in by 2 screws and a bolt. Slide the duct forwards out of the air box and remove.
Remove the air filter, then the 4 nuts that hold the air box to the AFM (metal box), and the 2 bolts mounting the air box to the car. carefully lift the air box out.
Cut the front of the air box out. Drill a starter hole in each corner, then cut using a jig saw or even a new hacksaw blade held with a handkerchief (this is really high tech here). You will need to use the hacksaw blade around the bottom anyway as a jig saw won't handle going around the duct outlet. Tidy the hole with a file, removing any rough edges.
Clean the air box out thoroughly, you don't want little plastic filings going through the engine. Refit the air box.
Cut a piece of coreflute or foam to form a side wall between the box and the engine bay and fit it in place. Mine just fits nicely and doesn't need anything to hold it in place. Cut and pack a small strip of foam to seal under the air box, and another two seal between the air box and the side of the car - I use a double thickness for that one.
Now cut a piece of foam to make a lid that seals across the front of the air box, the side panel and the front lip where the bonnet seals. I cut my foam with the foil side out, then painted the top lid black to make it look less obvious. Fit the lid, it should stay in place by being a firm fit.
You have now finished the first chamber of the BFI, which removes the restrictive duct. If you have a '31 or '32 you may want to construct the 2nd and 3rd chambers to get more air to the air box, as I have a UZZ30 which has no clutter below the headlight, I haven't yet bothered.