In the past I've used a Hypoxico generator to do hyperoxic training one of two ways:

1. Collect the 'waste' gas (~12 L/min of ~60% O2) into a 200 L meteorological balloon, then use that as a reservior mixed w/ additional room air 'on the fly' via a Y valve to obtain the needed volume; or

2. Use a nasal cannula to send the ~60% O2 directly into my nostrils.

Approach #1 provides more certainty regarding the exact final O2 concentration that is inhaled, but it can be a bit tricky to manage/balance everything just right so you can focus on training instead of running a science experiment.

Approach #2 makes the final O2 level a bit uncertain (since you don't know how much of the ~12 L/min is inhaled, vs. spills out of the nostrils during exhalation, and you also don't know the total ventilation) and can lead to nosebleeds (due to dryness of the gas), but is far simpler in execution.

Both approaches keep my SaO2 at 98+% during high intensity exercise, vs. the drop to 93-95% that I experience even during exercise at sea level (see below):