The easiest approach to upgrade an old site to 2010 is migrating it to a new 2010 farm, this is to avoid the unknown issues you may face because the upgrade process for the entire farm may require lot of updates to your config files and farm settings, and will drive you crazy if you did lot of customizations to 2007.
So before doing the upgrade process you have to install MOSS 2007 SP2 in your old farm, after that do below steps:
1- Backup your old site content database from the old farm using Microsoft SQL Management tool because this is the easiest way, you can detach and attach the database but doing backups is always safe.
2- Restore the database to the new farm.
3-Create new application under your SharePoint 2010 farm and remove the database SharePoint creates for you , it doesn't matter if you created a site collection or not it doesn't matter the site collection template also if you created it because we are going to remove the database.
4- Before doing this next steps it will be nice to know if SharePoint 2010 will accept your database or not, just do quick check if the database is ready for migration or not, by running “SharePoint 2010 Management Shell”, you can find this console window by using
Start menu –> Microsoft Sharepoint 2010 Products –> SharePoint 2010 Management Shell
Type: Test-SPContentDatabase -Name YOUR_DATABASE_NAME -WebApplication http://YOU_RAPPLICATION_URL:8090
5- if everything is ok, now we can upgrade the database by running below command
stsadm -o addcontentdb -url http://YOU_RAPPLICATION_URL:8090 -databasename YOUR_DATABASE_NAME
6- if you got an error telling invalid object “Sites” or Invalid Object “Webs” probably you didn't install MOSS 2007 SP2 in your old farm.
7- Maybe the database upgrade will run successfully and the upgrade will be completed but with some errors, in this case SharePoint will let you know where you can find the log file to start investigating your issues, it will show message like below one.
Upgrade completed with errors. Review the upgrade log file located in
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\LOGS\Upgrade-20100227-134745-781.log.
The number of errors and warnings is listed at the end of
the upgrade log file.
by following the error messages you can identify what was wrong during the installation and solve them one by one, however most of the errors occur due to a heavily customization in your 2007 site and installing 3rd party or custom developed features. by finding those features and installing them you will avoid lot of the issues.
