Dave's Redistrict, as already said, needs Silverlight to run. Chrome does not support silverlight, and I personally use Firefox. However, recently Firefox dropped their support (they said they may re-enable it) so I use the Extended Support Release of Firefox meant for web devs that runs pretty much everything. It can be found
here. Do be warned that DRA is a legacy product now, and does have a few memory leaks.
Custom party and also custom population (I moved most of the DC metro into the NYC metro for example) are all done through personal tabulation and spreadsheets. DRA gives you party vote for 2008, which is as good a baseline to have. From there I compare with both previous elections and the UK party coalitions. Other good cross references are income for the larger metros like Chicago, Atlanta, and NYC because they help tell where a rich Tory district, a poor Labour, or a international Whig district would fall. Race is also important - Blacks I made almost universally Labour. Hispanics are not as frequent nor as prevalent in my world, so I often mentally substitute them for more British migrant groups like Arabs, Indians, or in the case of Florida, International retirees. A general knowledge of what the counties background is (working class, suburban, city, poor, rich, minority, vacation economy, university campus, etc) also helps. Finally, I also have my parties have geographic bases: Whigs in the NE + East Florida, Tories in South + Suburbs, and and Labour in NW and Black areas. These all help in producing my maps.
I have also changed a couple regions, such as having Boston be a Silicon-Valley tech area thanks to the Whig policies and mass of universities.