Siebel BRY 96 (the strain that Sierra Nevada uses) is the absolute easiest yeast on the planet to culture from a bottle of bottle-conditioned beer. In fact, it was the first yeast strain that I cultured from a bottle of bottle conditioned beer. If you are going to culture in the bottle, you are going to need a sanitized funnel and a sanitized #2.5 solid stopper, which will be used to cap the bottle while shaking.
Procedure (assuming that the beer has been resting upright for long enough that the yeast has settled)
Remove the bottle cap
Wipe the mouth of the bottle with a 90% alcohol saturated cotton ball (you need to use a light touch because the goal here is to clear the surface of any dust that may have gotten under the crimped portion of the crown, not drench the bottle)
Flame the mouth by quickly passing the flame from a butane lighter over the mouth of the bottle (Charlie P. referred to this step as “burning the lips”)
Carefully decant the beer leaving the sediment and about 1/4" to 3/8" of beer.
Add 50 to 60ml of 1.020 wort to the bottle using a sanitized funnel (do not attempt to rush the culture by using more wort)
Cap the bottle using a sanitized #2.5 rubber stopper
Place your thumb or the palm of your other hand over the fat end stopper and shake until the liquid is nearly all foam (you should hold the stopper in place; otherwise, Murphy will more than likely show up, and you will have wort on your ceiling)
Remove the stopper and cover the mouth of the bottle with a piece of aluminum foil that has been wiped down with alcohol.
Allow the culture to incubate 24 to 48 hours before stepping (discard the culture if you do not see activity within 48 hours)
Step the culture to 250ml using 1.040 wort
Incubate the culture for another 12 to 18 hours
From this point forward, you can treat the culture as if you were making a starter with commercial yeast. If you are meticulous about sanitation, this process should produce a clean culture almost 100% of the time.
One last thing, yeast cultured from a bottle of SNPA behaves differently than Wy1056 and WLP001. Those cultures have drifted from the source. Cultured SNPA yeast is usually much more flocculent than either Wy1056 or WLP001.