What to listen for
An open hi-hat is the two cymbals of the hat stand struck while they are apart, so the sound rings instead of stopping dead. Samples range from a short half-open “tss” to a long, washy decay. Drum machine opens, such as the 808’s and 909’s, are bright and noisy; acoustic opens carry more pitch and a slower decay.
Pick the length for your tempo. At 125 BPM an offbeat open hat has a little under a quarter of a second before the next kick, so a long sample will smear unless you shorten it with the amp envelope.
Fitting it in the mix
Open hats live in the same band as vocals’ sibilance and snare crack. A high-pass around 300 Hz removes low thud you don’t need, and a touch of velocity variation keeps a sixteen-bar loop from sounding like a copy-paste. Choke it with your closed hat, as a drummer’s foot would.