Very Carefully.
But seriously, I don't have a place for a vice or anything of that sort in my small apartment, so here's what I do. First, I have two sets of needle-nose pliers. But not just any needle-nose pliers: these both have round holes in them, on the side, like this set:
http://www.istockphoto.com/file_thu.../2/istockphoto_1743785_needle_nose_pliers.jpg
It just happens I have 2 pliers on which the hole is just perfectly sized to hold an Aixiz housing in that little hole when the handles are squeezed together. In fact, one set gets jammed shut on the housing and won't open again without the same considerable force it took to jam them, which keeps the Aixiz from falling out. They are perfectly sized, so they also don't mar the finish on the housing.
So, I lay a pair of pliers on a flat, hard surface with the hole facing up, put the front of the the Aixiz housing in the hole, and 'lock' it onto the housing. I then place the diode in the hole, and use the other set of pliers, again utilizing the hole in the side of the pliers as a means to hold the back of the Aixiz housing. Using the "back" of the Aixiz housing, I apply a constant, even force downward on the diode, which is fairly easy to do with the pliers being closed all the way and providing a wide, flat area on which is press from above. So basically, the two pliers act as a vertical press, and the holes in the sides of the pliers keep the Aixiz housing properly aligned and positioned with much less risk of slipping off or breaking something.
This hasn't failed me yet, just make sure the diode is straight, and you press evenly and SLOWLY, and use larger things to be able to apply this even pressure.