I always used to use an old gear cable to break the master link. If you're just doing it every now and then it will be fine, don't worry about the tool.
Sizing guide:
Put chain in big ring on front and back, and pull the chain together as close as it can go. At this point your derailleur hanger will be pointing forwards (it will prove quite awkward to actually do this part). This is the absolute shortest your chain could be without causing issues... it makes sense right? This is the longest run the chain would ever need to make, and you're pulling the chain to it's shortest possible length at this point! Any shorter and it couldn't get into this gear and you risk breaking things if you accidentally try to.
Don't make it as short as possible though, give yourself a little bit of leeway. Release it a couple of links and break the chain there.
IMPORTANT! There are two types of link in every chain (think inside and outside). Make sure you are breaking the right one. Fortunately if you've done it as above and you mess up you can whip out that last link and you'll still be okay
oh also, make sure you've threaded it correctly before shortening it
Maybe post a picture up here of what you think is correct before going ahead and using the chain tool to break it.