I received some healthy Zinnias from Dobies with instructions to transfer
the plants to 3.5 inch pots using multipurpose compost etc.

I did this, but now 4 days later the plants now look sorry for themselves
and have started to wilt and turn brown ! What have I done wrong ?

To clarify what I did:
Placed multipurpose compost in pots, made hole and transferred plants
carefully loosening part of the root before placing in pots.

Quick watering with (Waterbutt) rainwater.

Transferred to (unheated) brick garage in front of a window that doesn't get
direct sun.

Any info would be appreciated as I am about to transfer some healthy Dahlias
into pots and don't want similar happening.
Thanks.

Mike.