The most important part has to be planning. You, as a host need to know exactly want you want the level to be.
First you have to find a song. After that you'll most likely want to figure out the style of the level. Is it an art-style level? Or a glow-style level? Of course you could let every part builder decide themselves, but that could lead to some weird levels that don't feel like a finished thing.
Now you will need a template for the level. Create a new level and and mark speeds, parts, etc. Give every part its own groups and color channels. That way it won't be annoying fixing every color afterwards.

This is how my template looks like. Try to give as much info as possible.

Now it's time to find builders. Explain everything to them. Don't forget: You need to communicate!

Option 1
The faster variant (have to use a hack).
Give every single player the level at the same time and let everyone build their part on their own template.

Option 2
Give the template to one of the builders. Let them do their part and then pass the project to the next builder until every part is finished.

Now after everyone has finished their part on their own template let them all upload the levels. Now your job eould be to put the level together.
Geomtry Dash has an annoying limitation, that you can copy only up to 100 objects between levels. Now you will need a hack to bypass this limitation. Also a hack to bypass object limitation if needed.
With this hack you will copy every single part to your clipboard and then paste all the parts in their own places on your own template.

https://m.youtube.com/watch?feature=youtu.be&v=-9Z6vJ0z18I
The hack. Pretty sure it has no virus, but I can't guarantee.

Is there anything else to say?

🔥 UPVOTE?
🔥 SHARE?
🔥 FAVORITE?