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

If a spell is copied, choices made while casting it are copied, so the copy will have the same abilities spliced onto it as the original.

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, including those from cards spliced onto it.

If the spell is countered or otherwise fail to resolve, any cards you spliced onto it remain in your hand.

Splicing Lifening Elemental onto an instant or sorcery will give it lifelink. It won’t make that instant or sorcery spell a creature spell, nor give it a power or toughness.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.