This Instructable covers the concept of the laterally sliding board, to be edited to fit your personal needs.
Please note: Freebord is a brand of freeboards (Lateral sliding boards) based in America. But the concept has been around since the 80's. The schematics are available online somewhere, check google.
Another brand is Gravitis Freeboards based in Italy.
Freebord makes great introduction Videos though, so:
The basic principle of the board is: Six wheels, One board. Four outer wheels let you carve just like a longboard. Two center wheels on casters let you slide.
In this example I made the board as well but won't cover that in this Instructable. My board is 100 x 20 cm with a very slight concave and flex.
1. I am now an Version 2.1. In this Instructable you will see some photos from the Latest Version as well as 1 and 2
2. The Board: for beginners it should be completely stiff. Flex or a camber will get you hurt . Concave is good for turning.
3. Wheels. I haven't given exact measurements for the casters or the wheels. it will depend on what is available to you. as a general rule you want to have small wheels, closer to the ground= more like snowboarding.
4. Tuning. After i was done i added riser pads to my casters in order to slide easier and wedges to the skateboard trucks to improve turning geometry. See the last page of this instructable