There are a handful of usual conditions for a plenary indulgence:
sacramental confession - at least within two weeks before or after saying the indulgenced prayer
communion, prayer (an Our Father, Hail Mary and Glory Be) offered for the Pope’s intentions, and the indulgenced prayer, all on the same day
the biggie - total detachment from all sin, mortal and venial. There’s a bit of debate about what exactly this means, but at the least you should, at the time of saying the prayer, have no weakness towards any habitual sins of any kind. In any event, prepare yourself for the indulgence by asking God to grant you the grace of detachment from all sin - I’m sure that’s something He really wants all of us to have, so He probably won’t deny it if you earnestly seek it.
As for your prayers, you may want to check them against the Enchiridon of Indulgences (you can find it online), there’s a 1986 edition (I believe) which is the most up-to-date record of indulgenced prayers.
I know that at least one can get a plenary indulgence any day for simply reading scripture using that lovely old Bible of yours for half an hour … so why not do that instead?
Yah, but that one has to be done in a church to attract the indulgence, as do the Rosary (unless you pray in a group) and the half-hour of Adoration of the Blessed Sacrament. The scripture reading seemed to be the obvious choice because she has the Bible to hand and can do it anytime anywhere.
So if I do 1, 2, and 3, those are the usual conditions. Then read the bible for a half an hour I will get a plenary indulgence? Communion is required on the same day? This seems so easy to wipe away temporal punishment, why haven’t I heard of this before?
I don’t know why more people haven’t heard of it - but the ‘total detachment from sin’ isn’t necessarily as easy a hurdle as you think, which is why I called it a biggie.
St Phillip Neri was officiating over a group of thousands of people who were trying to obtain a particular plenary indulgence. God reportedly revealed to the Saint that only he himself and one other out of the whole crowd had actually had sufficient detachment from sin that they’d attained it.
In any event, if you attempt a plenary indulgence and lack that condition of detachment, you will have gained a partial indulgence at least, so at least the effort is never wasted.