I see that one reply stated to put your mask skirt under the top of your hood so that you can blow some warm air into the hood. That’s good advice, but a lot of the commercially-made hoods have a cutout for the face that leaves a lot of the forehead uncovered. Check to see whether this is the case with your hood. If so, cut the commercial stitching off the hood’s top to expose the neoprene underneath. Get a sheet of foam neoprene (you don’t need much, or even old discarded suit material will do). Cut a piece so that you can glue it into the area you cut the stitching out of. Make sure it is a piece that comes down far enough to cover your forehead completely. You should also have it cover your face on the side too. The opening should come up onto your chin so as to cover everything except your lips. This way, you have thick neoprene on all the major heat loss areas of the head.
Your mask should not be a close-fitting one, but rather a larger one like the older 3-windows masks that have a larger air pocket. The close-fitting masks were developed initially for those who were breath-hold diving and didn’t wont to equalize a lot of air space, but that sacrificed the air pocket around the face and allows more heat loss from the face. The mask needs to be under the hood’s neoprene so as to not leak too, so carefully fit it before entering the water.
Tucking the hood under the jacket is good, but is a compromise. Years ago, when I got custom wetsuits from Bill Herder, we went with suits without zippers and a hooded vest rather than hoods. A hooded vest prevents water from entering the suit by following the hood’s seams down inside the suit. Some liked using a jacket with an attached hood, that was a “pull-over” design; if there was a zipper, it was upside down and started at chest level and worked down to close the suit. We also had beavertails on the jacket so as to preclude water from entering from the waist.
I hope this helps.
SeaRat