Find a bug to work on
Let's assume you didn't have the foresight to start working in a separate branch.  That's okay, you can quickly move your changes into a separate one with the following commands:
{{{
bzr branch ~/bzr/ubiquity ~/bzr/ubiquity.956531
cd ~/bzr/ubiquity.956531
(cd ../ubiquity; bzr diff) | patch -p0
}}}

Make sure you have the following in `~/.devscripts`:
{{{
DEBCHANGE_RELEASE_HEURISTIC=changelog
DEBCHANGE_MULTIMAINT_MERGE=yes
}}}
This will ensure that if the most recent version of ubiquity in `debian/changelog` has already been tagged as released, `dch -U` will create a new version.

Now, check to make sure you have the changes in the new branch, then:
{{{
dch -U "Lazily unmount partitions in the check for a Windows partition
to install Wubi on when we have a full partition table (LP: #956531)."
debcommit
bzr push lp:~ev/ubiquity/956531
# If you want to make additional changes, save yourself from having to run bzr push after each.
bzr bind lp:~ev/ubiquity/956531
# Ignore warnings about not being able to find api.launchpad.net, and try again.
bzr lp-propose lp:~ubuntu-installer/ubiquity/trunk
}}}

You may want to also `bzr revert` the changes you started with in your ubiquity trunk branch, if any.