Installing the mainboard is done traditionally. In Win has placed small bumps to screw the board unto. While this saves you the step of having to put in seperate headers, you also run the risk of shorting your board out in some situations. My suggestion for a fix is simple: use normal tape as an insulator and cover them up first, then screw the mainboard down. Once it is in there, it may become apparent that you are working with a fairly compact interior, even though the Allure may look rather bulky on the outside. While a long GPU should fit nicely, as it will go just past the underside of the hard drive bays, it is will be crowded enough even without such a unit filling things up.
Luckily installing the hard drives is easy no matter what else is inside the Allure. Just flip the cage out, screw the drive into place and swing it back into the chassis until it snaps into place. This is an excellent method for such a small case and I would like to see more manufacturers of mATX enclosures offering such a solution.
To install the optical drive, you will have to pop off the front cover first, open one of the drive bays up and then slide the device into place. Once it reaches the required depth inside the Allure, just push the the locking "buttons" down. This actually holds the drive rather well, with minimal play. For those who want to take the case on the road, I would still suggest using traditional screws as well.
To install a power supply you have to remove the entire rear panel. There is no mechanism to pop this one off, so you have to bend the clips back by yourself. Luckily In Win has made these rather long, thus there is minimal strain and they are easily bendable. Once the rear is taken off, the PSU can be secured with normal case screws.
After all the parts are installed and the PSU cables connected it becomes aware how crowded the chassis is. I could use zip ties to group unneeded cables and hide them behind the 3.5 inch bays, but In Win has not included any. So this is what the interior will look like if you do not happen to have some of your own lying around. For a case like this, I would suggest a modular PSU anyways. This should reduce the overall cable mess and has the advantage that most such unit come with sleeved cable as well.