An NBA playoffs pool can be scored two ways: by series (you predict who wins the best-of-7 tie, sometimes also in how many games) or game by game (you predict every single game of the series, just like in the regular season). Both are valid; they differ in how many predictions there are per series and how much attention they demand each week.
In the regular season, each team plays a single game against a different opponent almost every night: 82 chances to get it right or wrong, spread over six months. In the playoffs, though, the same two teams face off between 4 and 7 times in a row in the same series, so there are far fewer games in play at any given moment, but each one matters much more: a series can come down to the final game after going 2-2 or 3-3.
The simplest format for a playoffs pool is asking, before each series starts, who you think wins it and, optionally, in how many games (4, 5, 6, or 7). It's common to award more points the more precise the prediction: for example, 1 point for just picking the series winner, and 2 or 3 if you also get the exact number of games right. Since there's only one prediction per series, and a series can last up to three weeks, this format demands very little weekly commitment compared to a regular-season pool.
The alternative is to treat every game of the series like a normal pool game: you predict the score before each of those up to 7 games and score points just like in the regular season. This demands more attention — each series can produce between 4 and 7 predictions instead of just one — but it better rewards whoever follows the series game by game instead of betting everything on a single initial prediction.
If your group already had the pool running throughout the regular season, it's most natural to keep the same game-by-game system for the playoffs: no need to explain new rules or pause the accumulated standings halfway through. If instead you're only joining for the playoffs, a simple series-based pool demands less follow-up while still giving you the "bet you can't call who advances" banter.
Match Prophets scores game by game, including playoff games: every game of a best-of-7 series is predicted and scored separately with the same 3/2/0 system used in the regular season for the NBA, so there's no need to build a separate series pool — you just keep predicting in the same league until the trophy gets lifted.