Trying to eat the leaves on their own is a horrible idea. If you really want to try it (I suggest you don't), you'd be better off mincing and crushing them.
Bioavailability of taxine B, the chemical that actually does the work, is quite high. Toxicological publications estimate that you need 0.6-1.3 grams of leaves per kilogram of your body weight in order to experience severe life-threatening effects; you'd likely want to go overboard and do at least double that to be sure.
The problem is that you don't have a way to know how much poison a yew tree's leaves actually contains. They're the most poisonous during the Winter, from November to January. Do you really want to take that gamble?
Consider the symptoms of yew poisoning. You're administering it through oral ingestion, so that means it doesn't instantly hit your bloodstream. You need to wait for hours as it works through your digestive system, during which the symptoms gradually ramp up, and your body goes on high alert to prematurely purge the poison. You'll be extremely delirious, shitting and pissing yourself, constantly vomiting, suffering from seizures, phasing in and out of consciousness, seeing shapes and lights-- and all of this for a very long time before the lethal symptoms even appear.
Fasting, like with most orally ingested drugs, helps them take effect quicker and is a good idea, but you'd need an extremely heavy dose of sedatives (barbiturates or research-grade benzodiazepines) to knock yourself out so you're not conscious for the torture, and antiemetics to prevent vomiting it all out, which WILL happen (and even then, it'll try to expel it through your feces and urine instead).