Must I see a dream? No. Nothing in the hadith mentions dreams. The sign is that Allah makes the good path easy and turns the harmful one away.
When can I pray it? At any time it is permitted to pray voluntary prayers. Avoid the disliked times (right after Fajr until the sun rises well, at midday zenith, and after Asr until sunset).
Can I make the dua without praying? If you are unable to pray — such as a woman in her menses — the scholars permit making the supplication alone, though the complete Sunnah is the two rak'ah with it.
What if I still feel unsure? Repeat it, keep consulting sincere advisers, then act. Acting after istikhara with trust in Allah is itself part of the Sunnah.