To add a digital signature block to your proposal, click the + icon located at the bottom of any block and click 'Digital Signature'

This will add a digital signature section to your proposal. You can only have one per proposal, so if you added one already, then you'll see a message to let you know that you can't add another one without removing the other.

Don't worry if you need multiple people to sign the proposal, you still only need to add one block at this stage, you'll add your signers later.

You'll notice that the {{first_name}} and {{surname}} merge tags are in the acceptance text by default. You don't need to do anything with these as they will be automatically replaced with your client's name when they open the proposal.
Bear in mind you also won't see these replace on the proposal preview, because it's not tied to a specific recipient.

By default, your recipients will be able to choose whether they type their name to sign, or draw their signature. You can change which of these options is available to them (either one or both).

And a nice added touch allows you to change the background colour of the signature block. This doesn't change the colour of the box itself, just the background behind it, so you can use this to blend it into the rest of the proposal if you're already using background colours.