1. Explain it simply
一起 means 'together'. Put it after the subject and before the verb: 我们一起去…
Add 吧 at the very end to turn it into a soft suggestion: 'let's…'.
Use 吧, not 吗 — 吗 would make it a plain yes-no question instead.
2. Show examples
我们一起去踢足球吧。
Wǒmen yīqǐ qù tī zúqiú ba.
Sandhi:Wǒmen yìqǐ qù tī zúqiú ba.
Let's go play football together.
一起 before the verb, 吧 at the end
我们一起吃饭吧。
Wǒmen yīqǐ chī fàn ba.
Sandhi:Wǒmen yìqǐ chī fàn ba.
Let's eat together.
A friendly suggestion
你们一起来吧。
Nǐmen yīqǐ lái ba.
Sandhi:Nǐmen yìqǐ lái ba.
You all, come together!
一起 stays in front of the verb
3. Share a tip
一起 sits right before the verb, never after it. Ending with 吧 makes a suggestion; ending with 吗 makes a yes-no question.