In your Workflow project, goto the Publishing tab and set the Workflow Type to "Webservice". Then in the Project Tree, under the Primary Model look for "Input Data". Click on that and then on the right side, add your variable names and their datatypes you want to pass from the WebForm project to the Workflow project. See the example below with one variable defined.
Publish your Workflow to your server. Then you need to create an integration project (if you don't have one already) and add a Web Service Caller generator that points to the URL of you Workflow webservice. It will be in the form of http://<your server>/<workflow virtual directory>/MyService.asmx. (Note you can change the name of the webservice asmx inside the Workflow project if needed under the Publishing tab where is has the Primary service section). Comppile and close the integration project.
Open your web form project and add the integration project library to that project. Then in the component toolbox drag and drop the Execute Component from your web service to the point in your web form you want to pass over to your workflow. Edit the component and map in the data to pass that corresponds with how you setup the 'Input Data' in the workflow I mentioned previously. Debug to test.
That's it in a nutshell. Here is a good video from the WorkflowSWAT site that explains how the Web Service generator can be configured.
Enjoy!