Go to the 'Send' tab and paste your invoice. 'Description' field will be automatically filled since this information is usually provided in the invoice. Note that you will not be able to send all of your coins. Each channel must maintain a reserve for its closure fees. You can see how much you can send from each channel by going to the 'Channels' tab, right-clicking on a channel and selecting 'Details...'. You cannot send from multiple channels at once.
Does anyone know if there is currently a convenient way to choose which channel you want to be used to send a LN transaction, when you have several channels opened, without having to
freeze all the other channels during the transaction please? Same question when you need to receive a payment through a generated invoice : is there a way to receive the payment on a specific channel without freezing all the other channels until you receive the payment?