Before migrating

Update yourself with the latest OCA Conventions: https://github.com/OCA/maintainer-tools/blob/master/CONTRIBUTING.md
Subscribe to the mailing list of the related project: https://odoo-community.org/groups
Announce on the corresponding GitHub issue with the name "Migration to version 11.0" which module(s) you want to migrate

Tasks to do in the migration

Bump module version to 11.0.1.0.0
The first line # -*- coding: utf-8 -*- is not needed anymore in Python 3 if you save it in that encoding. (You can use the following command to auto-delete it: find . -name "*.py" -exec sed -i "/#.*coding\: /d" {} \;)
Update README.rst from https://raw.githubusercontent.com/OCA/maintainer-tools/master/template/module/README.rst if not updated to the latest template.
Convert Python 3 incompatible code. You can automate some things using 2to3 utility (bundled in most Linux distributions) with this command being on the module directory (it can require a later manual review for optimizing some sentences):
```
2to3 -wnj4 --no-diffs .
```
You can also check Python 3 compatibility and conversions guide by Odoo: https://github.com/odoo/odoo/blob/11.0/doc/python3.rst
Remove any possible migration script from previous version.
Remove the use of workflows (they have disappeared in this version).
All area configs have been merged on a general res.config.settings model, so you have to adapt your possible settings in your module.
For v10 ir.values entries:
- if they are for showing an option under "Print" or "Actions" dropdown menu, remove the record and just add a field binding_model_id on the ir.actions.act_window linked record. Add binding_type = report if the ir.values had key2 = client_print_multi.
- if they are for having default values, use instead the model ir.default.
If you use reports, report module has been split between base and web:
- Change all references of ir.actions.report.xml by ir.actions.report, as the model has been changed.
- Replace t-call occurrences of base report templates:
  - report.external_layout > web.external_layout.
  - report.external_layout_header: No direct equivalent. You need to insert the changes inside div class="header o_clean_header"> element of the web.external_layout_? view, being ? one of the available "themes" (background, boxed, clean and standard in core)
  - report.external_layout_footer: the same as above, but looking inside <div class="footer o_background_footer"> element.
  - report.html_container > web.html_container.
  - report.layout > web.report_layout.
  - report.minimal_layout > web.minimal_layout.
Remove group_ids field in ir.config_parameter records, and read/write them with sudo() instead (github.com/odoo/odoo@d0ca2d115e).
If using widget kanban_state_selection in any view, change it to just state_selection.
If you create ir.cron in module setup you have to change:
- model (string) -> model_id (m2o to model)
- function (string) + args (string) -> code (eg: model.cron_that_does_something())
colors attribute in <tree> tag has been replaced by decoration-* (warning, info, danger...) attributes (bootstrap codes) with the condition as value for each.
Add tests to increase code coverage.
Do the rest of the changes you need to do for making the module works on new version.

Howto

Technical method to migrate a module from "10.0" to "11.0" branch

$REPO: the OCA repository hosting the module
$MODULE: the name of the module you want to migrate
$USER_ORG: your GitHub login or organization name

Full process for beginners in Git flows

On a shell command:

$ git clone https://github.com/OCA/$REPO -b 11.0
$ git checkout -b 11.0-mig-$MODULE origin/11.0
$ git format-patch --keep-subject --stdout origin/11.0..origin/10.0 -- $MODULE | git am -3 --keep

Check https://github.com/OCA/maintainer-tools/wiki/Merge-commits-in-pull-requests for a procedure for reducing commits from "OCA Transbot...".
Adapt the module to the 11.0 version following tasks to do.

On a shell command:

$ git add --all
$ git commit -m "[MIG] $MODULE: Migration to 11.0"
$ git remote add $USER_ORG git@github.com:$USER_ORG/$REPO.git # This mode requires an SSH key in the GitHub account
$ ... or ....
$ git remote add $USER_ORG https://github.com/$USER_ORG/$REPO.git # This will required to enter user/password each time
$ git push $USER_ORG 11.0-mig-$MODULE --set-upstream

Short method for advanced users that know the rest of the Git flow

$ git remote update # In case the repo was already cloned before
$ git checkout -b 11.0-mig-<module> origin/11.0
$ git format-patch --keep-subject --stdout origin/11.0..origin/10.0 -- <module path> | git am -3 --keep

Troubleshooting

Sometimes, when performing these operations, the process can hang due to conflicts on the patches being applied. One of the possible problems is because a patch removes entirely a file and git am is not able to resolve this. It ends with a message error: ...: patch does not apply.

If that's your case, you can add --ignore-whitespace at the end of the git am command for allowing the patch to be applied, although there will be still conflicts to be solved, but they will be doable through standard conflict resolution system. Starting from the previous situation, you will need to do:

git am --abort
git format-patch --keep-subject --stdout origin/11.0..origin/10.0 -- <module path> | git am -3 --keep --ignore-whitespace
# resolve conflicts (for example git rm <file>)
git add --all
git am --continue

Initialization (already done in OCA)

Before migrating the first module, the following tasks must be performed in the repository:

Create empty 11.0 branch.
Create metafiles (.travis.yml and README.md) from 10.0, adapting them to have the proper data.
Make 11.0 default branch.

There's an script to automate all these process in: https://github.com/OCA/maintainer-tools/blob/master/tools/migrate_branch_empty.py.

Some repositories PSCs have decided to keep previous procedure putting installable flag on module as False, using this script: https://github.com/OCA/maintainer-tools/blob/master/tools/migrate_branch.py.