You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell.

A card with a splice ability can't be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.

You choose all targets for the spell after revealing cards you want to splice, including any targets required by the text of any of those cards. You may choose a different target for each instance of the word "target" on the resulting spell.

If all of the spell's targets are illegal when the spell tries to resolve, it won't resolve and none of its effects will happen.

With the introduction of the kindred type it became possible for an instant or sorcery card to share a type with the sacrificed permanent (ie, a kindred instant card shares a type with a kindred enchantment card). Reweave has received errata so that the effect looks for a *permanent* card that shares a type with the sacrificed permanent. A permanent card is a card that could be put onto the battlefield, which is any card that isn't an instant or sorcery.

The player reveals cards only if the targeted permanent gets sacrificed.

When Reweave resolves it uses the type(s) that the permanent had right before it was sacrificed.

Reweave : règles, fonctionnement et interactions (MTG)

Reweave is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target permanent's controller sacrifices it. If the player does, they reveal cards from the top of their library until they reveal a permanent card that shares a card type with the sacrificed permanent, put that card onto the battlefield, then shuffle. Splice onto Arcane (As you cast an Arcane spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)

How it works : Reweave

Original English text — Wizards of the Coast / Scryfall

#1June 7, 2013· Wizards of the Coast
You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell.
No other card is named in this ruling.
#2June 7, 2013· Wizards of the Coast
A card with a splice ability can't be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.
No other card is named in this ruling.
#3June 7, 2013· Wizards of the Coast
You choose all targets for the spell after revealing cards you want to splice, including any targets required by the text of any of those cards. You may choose a different target for each instance of the word "target" on the resulting spell.
No other card is named in this ruling.
#4June 7, 2013· Wizards of the Coast
If all of the spell's targets are illegal when the spell tries to resolve, it won't resolve and none of its effects will happen.
No other card is named in this ruling.
#5April 1, 2008· Wizards of the Coast
With the introduction of the kindred type it became possible for an instant or sorcery card to share a type with the sacrificed permanent (ie, a kindred instant card shares a type with a kindred enchantment card). Reweave has received errata so that the effect looks for a *permanent* card that shares a type with the sacrificed permanent. A permanent card is a card that could be put onto the battlefield, which is any card that isn't an instant or sorcery.
No other card is named in this ruling.
#6November 1, 2005· Wizards of the Coast
The player reveals cards only if the targeted permanent gets sacrificed.
No other card is named in this ruling.
#7April 1, 2005· Wizards of the Coast
When Reweave resolves it uses the type(s) that the permanent had right before it was sacrificed.
No other card is named in this ruling.