1) Install using web installer zip from here : https://github.com/wintercms/web-installer/releases/latest/download/install.zip
Step mentioned here : https://wintercms.com/docs/setup/installation#web-based-installation
Installer is at /install.html 

2) Make our zip from zip archive mentioned in the code in web installer folder i.e /install/install/api/src/Api.php
i.e in constant const WINTER_ARCHIVE = 'https://github.com/wintercms/winter/archive/refs/heads/1.2.zip';

3) Copy 'vendor' folder, 'composer.lock' file from manual installation folder to our zip. Check and copy composer.json as well from manual installation if there is a difference in downloaded zip's file  and installed file.

4) Check development dependencies in every version in this file /install/api/src/Api.php i.e in 'postCleanUp()' function,
i.e only the one's mentioned with comment //Remove core development files. Add those files to our <exclude> tag in install.xml.
These dependencies are removed by the script. Also compare with manual installation zip with original zip downloaded from https://github.com/wintercms/winter/archive/refs/heads/1.2.zip to 
check these changes and do them accordingly in our install.xml.

5. PHP REQUIREMENT LINK : https://wintercms.com/docs/v1.2/docs/setup/installation#minimum-system-requirements. Also check in install/api/src/Api.php in MIN_PHP_VERSION its mentioned 7.2.9 so we have kept the same in info.xml. Check PHP version on release notes as well for every major version release.

6. In config/cms.php, Change 'linkPolicy' => env('LINK_POLICY', 'detect'), to 'linkPolicy' => env('LINK_POLICY', 'force'), suggested by vendor as the image preview and the URL was breaking. Check in next version if the issue is resolved
https://github.com/wintercms/winter/issues/947#issuecomment-1872392255
Checked in 1.2.6 . Need to keep it to 'force', as image preview breaks.