No worries if you don’t have a sandbag! Use a dumbbell (both hands), plate or schoolbag. Make sure you engage your core throughout, and think about lifting straight up like an elevator (try not to lean forward).
Modification: simply lie down and stand up. If you’re having a lazy day, try it in bed! Fab back support too. :)
1, 5 or 10 minutes is all you need! I’m trying to break my old score of 42 in 10 minutes (aiming for 50). Keep score so you have a benchmark to beat for next time. :)