1. Explain it simply
既然 (jìrán) means 'since' or 'given that'; it points to a fact that is already established.
Word order: 既然 + known fact + comma + subject + 就 + conclusion.
The second clause is the logical result or a suggestion, and 就 sits before its verb.
既然 is not 因为 - it reasons from a fact both speakers accept, so it pairs with 就, not with 所以.
2. Show examples
既然天气这么好，我们就去公园吧。
Jìrán tiānqì zhème hǎo, wǒmen jiù qù gōngyuán ba.
Since the weather is so nice, let's go to the park.
既然 states the known fact
既然大家都来了，我们就开始吧。
Jìrán dàjiā dōu lái le, wǒmen jiù kāishǐ ba.
Since everyone is here, let's start.
就 draws the conclusion
既然你喜欢这件衣服，就买下来吧。
Jìrán nǐ xǐhuan zhè jiàn yīfu, jiù mǎi xiàlái ba.
Since you like this piece of clothing, buy it.
就 leads to a suggestion
3. Share a tip
既然 leans on a fact you both accept; keep 就 before the verb of the result, as in 我们就去, not 就我们去.