How can I use a 'plastic sled'?
You need to combine the sled base with 2x Medium Thread. Once you do you can equip the sled to pull it behind you, carrying more. However you will be unable to run with the sled.
The sled is considered a vehicle, so to equip it you need to place it in the vehicle slot.
In addition to placing the sled in the vehicle slot, you can equip the sled by right clicking and selecting Take (unless you already have a vehicle equipped).
Also, the sled can be taken as is (without a strap), but it gives the condition "Pulling sled awkwardly" which is an encumbrance penalty (source).