Istikhāra literally means "to seek that which is good" and is the third part of a 3-step decision making process:
Pray two rakaʿāt of voluntary prayer (any prayer outside the five obligatory ones) and recite any Surahs you'd like after Fatiha in each rakah as there are no Surahs specifed by The Prophet ﷺ. Once you finish - ie, you say your salām for the prayer - recite the duʿāʾ (prayer) of istikhāra in Arabic as recommended to us by the Prophet ﷺ and then explicitly state your need in any language you'd like. If you don't have it memorized, read it off a paper or your phone. If you can’t read Arabic, read the transliteration. No extra duʿāʾ afterwards is required.
You will NOT necessarily see a dream/vision/sign/etc. Allāh ﷻ will place a bit of inclination in your heart towards going through with the action or not.
Istikhāra is a natural process. Don't expect anything otherworldly. All istikhāra ensures is that you have the blessings of Allāh backing your choice. There is no istikhāra for Ḥarām actions (ex. Should I drink this alcohol?) or obligatory actions (ex. Should I pray ʿĪshāʾ today?). Do not regret your decision afterwards as doing so would be to regret and doubt in Allāh's guidance. Even if your decision "doesn't work out" how you envisioned, know that it was better for you and that there are blessings in it even if you can't see them yet.
Then know that God simply hasn’t placed that inclination in your heart yet. This could be a sign that God ﷻ wants you to keep begging Him and asking Him for His Divine Help and Assistance in this process. Keep making duʿāʾ. But never lose hope. Even if you asked a thousand times with no response, that’s no reason to stop asking.
There is no precedent for this. It's supposed to be done by the one seeking the decision.
Allāh is accessible to all. Even the greatest sinner can turn back and ask Allāh. This is a sign of your sincerity and trust in Allāh.
Then everyone involved should all be praying their own istikhāra.
Yes, because it'll increase the blessings in your actions. The companions [rḍA] would make istikhāra even for simple everyday things.
Just the duʿāʾ [supplication] is sufficient. The Prophet ﷺ explicitly said to perform the two rakaʿāt, however you may recite the duʿāʾ by itself if it’s something you choose to constantly do, or if you have an immediate decision. So for a sister who is currently not praying, it is completely valid and acceptable for her to only make this duʿāʾ without the physical prayer.
Say the duʿāʾ in English and know that Allāh understands your prayers to Him no matter what language it’s in. — And as always, pray to God from your heart. God wants to know that you are truly and sincerely begging Him and Him alone. If that’s best conveyed in a language other than Arabic, then sobeit.
Because The Prophet ﷺ is the one who taught us this duʿāʾ and just by uttering the very words he ﷺ uttered, we get extra blessings. Additionally, his amazing companions [rḍA] would say that The Prophet ﷺ taught them this duʿāʾ like he ﷺ taught them parts of The Qurʾān.
No. Istikhāra is the slave’s way of begging his Lord for blessings in his decision, but his Lord is far too Generous to make His Blessings dependent upon the actions of His slave.
The Prophet ﷺ didn’t specify a number for us. So whether it’s once or a thousands times is all based on the individual. If you still haven’t felt an inclination towards a decision, keep asking. If you did feel an inclination, know that that is from God and accept it. There is no need to keep asking to be “sure” of the istikhāra.