Configure all your Access Points so they have the same SSID, security/encryption type, passphrase, etc. Basically everything "wifi" needs to be the same except the radio channels.
You shouldn't need WDS.
You can have all AP's on the same radio channel, but thence they would "interfere" with each other and throughput would go down a bit. Set each AP on separate channels from the set 1,6,11 avoids this. And of course, the AP's coverage areas need to overlap.
Roaming decisions are made by the client, not the AP's. Some clients will doggedly hang on to an AP until it has to roam, some will constantly hunt for the best AP, some have a setting in the NIC called "roaming agressiveness" or something similar which allows you to exact a degree of control.
Thus far I've never seen it work entirely "seamlessly" - there's always a blip of a second or so. Fine if you're just surfing web pages, but just be aware of that if you expect to run around the house streaming video.