יְיָ אֱלָהָא דְאַבְרָהָם יִצְחָק וְיִשְרָאֵל אַבְהָתָנָא נְטַר נִדְבָתָא דָא לִזְכוּתָא לְעָלָם לְקַשָׁטָא יֵצֶר הִרְהוּרֵי לִבְבָא דְעַמָךְ וְאַתְקֵין לִבְּהוֹן לְמִדְחַל לְוָתָךְ.
1. keep this | שׇׁמְרָה־זֹּאת – In the context of the verse in Divrei HaYamim, "this" refers to the good will of the people and their many donations for the Temple. David asks that it be remembered for them as a merit. Cf. Avudraham that in the context of the prayer of "קדושה דסדרא", the word "this" might refer to the saying of the "Kedushah" (sanctification), with the connotation being that sanctifying Hashem should help inspire one's thoughts and heart towards God.
2. as the design of the people's hearts | לְיֵֽצֶר מַחְשְׁבוֹת לְבַב עַמֶּֽךָ – The translation assumes that, here, the preposition "ל" of "לְיֵֽצֶר" does not mean "to", but rather marks the object of the verb. David prays that just as the people proved themselves willing to donate to Hashem (as described in the earlier verses of Divrei HaYamim 29) so too in the future the people's hearts should always be inclined to do good (Malbim). Cf. Avudraham who reads "לְיֵֽצֶר" as a verb, "to form"; the people's deeds should form and inspire their thoughts. The phrase recalls Bereshit 6:5, where Hashem sees that the designs of man's heart are always evil ("וְכׇל יֵצֶר מַחְשְׁבֹת לִבּוֹ רַק רַע כׇּל הַיּוֹם") and decides to bring the Flood. David prays that Hashem's earlier assessment be turned upside down.
3. and direct | וְהָכֵן – The root "כון" has a range of connotations and can mean: "to be firm" or "established", "to prepare", or "to direct". Here, one might, thus, alternatively translate the clause as: "and make their hearts stand firm towards You".