If it has a change, spend it to perform manual CPFP on that transaction.
Add another input(s) if it doesn't have enough balance for the child transaction's fee rate.
Since CPFP works differently than RBF, you need to compute the right transaction fee.
To do that, you only need a simple calculator or pen&paper:
- Get the total fee and the virtual size (in Electrum: size) of your "final" transaction (parent), example: Fee: 0.00001000 BTC (not the fee rate), 200 bytes.
- Then create the child transaction using coin control feature to select the inputs (address/coins tab->right click->spend-from)
- Proceed to send tab, add one of your receiving address and open the "advanced preview", from there, check the child transaction's size, example: 150 bytes (keep the window open).
- Check the optimum transaction fee rate from your preferred sources, example: 30 sat/B
For the computation:
- Just add both of the transaction's sizes: 200 + 150 = 350 vBytes.
- Multiply the optimum fee rate by the total transaction size: 350 vBytes ∙ 30 sat/vB = 10,500 satoshi or 0.00010500 BTC.
- Subtract the parent transaction's fee from the above: 0.00010500 BTC - 0.00001000 BTC = 0.00009500 BTC
- Lastly, divide it by the child transaction's size: 0.00009500 BTC or 9,500 satoshi / 150 bytes = 63.33 sat/vBytes <-example.
Go back to the advanced preview and set the result's fee rate.