If your SharePoint 2007 is highly customized you'll most likely need to make changes to your target site (Master pages will not fit directly etc.) As you might know you can basically either migrate through a Database attach detach method or using a third-party tool.
For the first method, the link below might be really useful as it contains the correct steps to perform this operation as well as another links that will lead you to a more detailed procedure: http://sharepoint.stackexchange.com/questions/71177/migration-from-2007-to-2010-by-db-attach-approach
However as you'll notice this method has limitations such as broken links and workflows and could be time consuming. Normally using a third party tool is the preferred method but again YMMV. There are many tools on the market. I recommend you to test their free trials to know which one better suits your needs. Also, don't be afraid to ask them questions so you can have an idea of the support experience and also make sure that you are taking advantage of all the features.
I am a bit biased toward Sharegate because I work on it and with it every day but I simply want you to have a global perspective on what is offered so you can make an educated decision on the best procedure to migrate your 2007 environment. You can start you evaluation process here: http://en.share-gate.com/download
Stephanie from Sharegate