for my Evereset North Col trip, i did hypoxic training in the pool 3 times a week for 4 months prior the expedition. basically i would swim about 2 miles breathing normally one pool length and holding my breath the other pool length.
for the "no breathing" part i was simply swimming under water first, but pool management didn't like it much and said it was a big liability for them due to shallow water blackout danger. so i simply switch to freestyle swimming with my face constantly in the water and that fixed the problem.
that with addition to running (and weight lifting) helped me a lot with high altitude. or at least i want to believe it did