Okay. So first you need to find where the potato is at.
Go under The Spawnmenu tab Games->GarrysMod->Phx_Props->Misc (I think)
Then find the potato. Spawn it in by clickign it. Now turn on SV_cheats 1 in console
Press ~ button adn type in sv_cheats 1 and press enter.
Now type in console physgun_wheelspeed [big number] and press enter. A big number is a big number. I like to mash random numbers until it's longer than the bar.

geta multiplayer server and d the setps in preparing. Now pick it up then move the scroll-wheel dwon then up and release and it should go flying fast.
Now do it at soemoen adn hit them, they should die unless they have lots of health. Enjoy!
This is propkill btw k?
Havefun.