Go to the Fund's page and click "Create Proposal". Do not forget to connect your wallet.
Only Fund's manager and Fund members are able to create proposals. You can find the list of all possible proposals with the corresponding roles needed to create them here.
You will see a screen, where you can see all types of proposals that you can create.
Let's create a proposal to change rage quit as an example. Click on "Change rage quit fine" and enter the Proposal Name and Description on the left, which will be visible to all voters, and the proposed value on the right. Try to be short and precise, while describing the reason for the change to your investors.
Click on Create Proposal and submit the transaction in your Metamask. The proposal is now created, it may take up a couple of minutes, and then it will be seen on Fund's page.