Undermount Drawer Slide Installation
Installation of undermount drawer slides is about the same as the installation a center mount slide except for the fact that there are two slides. The best slides to use for under mounting are ball bearing slides, commonly used for side mounting. Keep in mind that these slides will have a lower weight rating when used this way.
Note: It will be necessary to use rear mounting brackets if you are installing under mount slides in a face frame cabinet. If you need help with these brackets click here.
1. Undermount slides should be placed as close to the edges of the drawer as possible, this provides more stability to the drawer.
2. Align the slides along the outside edges of the drawer and mark their corresponding location on the floor or face frame of the cabinet.
3. Fasten the cabinet members to the floor of the cabinet using the marks that you just made. Or, mount them on the face frame and rear mounting brackets. When using rear mounting sockets make sure to use a level.
4. Reattach the slide members and make sure they lock by pulling the drawer in and out several times.
(Source: http://www.slidedummy.info/umdrawerslideinstallation.htm)