Make sure your client is invited to iCouch. In your People section, if you see a "Secure Message" option near their name, that means they have created a client account. If not, you'll need to Invite them (or re-send the invitation.)
Go to your Papers section. Click the Blank Forms tab.
Select the form you want them to fill out by clicking the checkbox next to the title. Go up to "Actions" in the top right -- select "Share" -- then select your client. (Be sure your form has "require signature" checked at the bottom -- you do this when you're editing/creating your form in the Form Template Manager.)
They sign into their account, go to Papers and they'll see the form listed. They click it, fill it out, draw a signature at the bottom, then click "Confirm Signature."
They click Save and Submit at the top of the page and that sends a copy back to you -- the completed form will be located in your Papers section (not the Form Template Manager.) The client will also have a copy of the form in their Papers section. After it is signed and submitted, it can no longer be edited -- it's "locked."