This can be done in two ways, simply looking at your wallet on etherscan, or going to the transaction that locked up the tokens and identifying the token ID that was minted to your wallet representing your lockup plans.
For example, this wallet address has 1 lockup plan with token Id 72.
Go to the etherscan link of the contract address. Clicking into the top right where it says Token Contract, we can now visit the contract address on etherscan. Navigate the buttons to Contract, and then to 'Write Contract'.
Then click the "Connect to Web3" button. It should show your connected wallet once successful. To use a multi-sig Gnosis Safe, you can connect with Wallet Connect.
Navigate down to the function that reads "segmentPlan":
Then input the token Id into the planId field, for this example I'm using #72.
Next input the amount you wish to make a segment of. The amount input should contain all of the decimals, so for instance use "10000000000000000000000" (with the quotes) for 10,000 tokens. This function will create a segment of 10,000 tokens, and then the main plan will contain the remainder of 90,000 tokens. Then click the "Write" button. This will trigger a metamask transaction, or via wallet connect a request for signature in your gnosis safe.
You will see in the transaction that you are minted a new token plan, in addition to the one you have. Now you have two token plans, the original will have the original amount less the segment, and your newly minted token plan NFT will have the segment amount (in this example 10,000).