The phrase でよければ combines the case marking particle で with よければ, the conditional form of the い-Adjective いい. Its literal sense is 'if (A) will suffice' or 'if by means of (A) it's good', which closely mirrors the common translation, 'if (A) is okay'.

でよければ is always placed after a noun or a phrase that functions as a noun.

• ラーメンでよければ、駅(えき)の前(まえ)に人気(にんき)の店(みせ)がありますけど、行(い)ってみませんか。
  If ramen is okay, there's a popular shop in front of the station; would you like to check it out?
• この古(ふる)い辞書(じしょ)でよければ、どうぞ使(つか)ってください。
  If this old dictionary is fine, please feel free to use it.

でよければ is frequently used when referring to the speaker themselves, serving as a humble way to propose assistance, suggestions, or one's own services.

• 私(わたし)でよければ、いつでも話(はなし)を聞(き)くよ。
  If you don't mind talking to me, I'm always here to listen. (If I'm an acceptable person to talk to, I'll listen.)
• 先輩(せんぱい)でよければ、僕(ぼく)が仕事(しごと)のやり方(かた)を教(おし)えますよ。
  If you don't mind learning from me, I can show you how to do the work.

Because で retains its core function of marking a means or method, でよければ is used to ask if (A) is an acceptable option for achieving a particular goal.