There’s a lot of complete BS trying to pass itself off as key lime pie out there. Here’s how my family always did it, although to be fair, the eggs used to be raw.
For suggested tools, a spice grinder (not as in mortar and pestle, but the one that looks like a tiny version of a cheese grater) is excellent for grating the key limes. Also, glasses are usually enough for eye protection, but a cheap pair of goggles from Home Depot may help those that don’t wear any.
If using a pastry crust, cook it for about seven minutes at 350° F between two pie pans (for the sake of maintaining its shape).
Separate eggs. If you aren’t used to meringues, try not to get any egg yolk into the white, as it will make it very difficult to beat to stiffness.
Grate the rind of about five key limes into the egg yolk, then begin to juice them into the yolk. You may want eye protection, as while key limes are sweet, they are still very acidic. I usually slice them down the middle and squeeze each half from each side, like into a cocktail; but I’m sure there are many ways of doing it. Afterwards, empty the can of condensed milk and the brown sugar in, and beat on high until homogeneous.
Clean the beaters with running tap water before making the meringue. Add the brown sugar to the egg whites, along with the meringue, and beat on high until stiff peaks form that do not collapse when the beaters are removed. This may take five minutes or more, depending on your beaters.
Pour the lime custard into the pan, and bake it at 350° for about five to seven minutes. Remove from oven, and add the meringue, being careful to seal the custard into the crust with it. I usually make it a point to coat the pie first, then get aesthetic, as the meringue is technically acting as the second crust. Return the pie to the oven for about seven minutes, until the top of the meringue is little crisp and very lightly browned. Remove, and allow to cool. Best served chilled.