Footprint/groundsheet/carpet - don't understand!

[Veggy]

I have an old canvas cabanon tent that has no ground sheet sewn in (except in the two "bedrooms". I need advice on what to use to cover the ground in the central communal part the tent - so it needs to be waterproof and a bit cosy.
We bought the tent second hand and it did have a very rough loose ground sheet that we used for a couple of years but it was really coarse and horrid to sit on. Then one of the kids peed on it and it stunk so I threw it out.
I went away last weekend and bought a cheap bit of tarp but it just rucked up, slid around and was useless.
Does anyone have any suggestions please. And, I don't want to spend a fortune and car space at a premium (!) but I'll be spending two weeks in the tent so I want it to be good and useable again for a few years.
Also - what do you use on the floor under the canopy - will a pic nic blanket do?
Thank you!

Ground sheet or tarp, with picnic blanket over the top. Can you peg the tarp down so it doesn't slide around?

Someone suggest damp proof membrane? But I agree it will need to be pegged down.

Do you know the measurements of the area? You can get porch groundsheets or groundsheets for trailer tents that might be the right size.

What about a tarp but get an eyelet kit to make holes to peg it down? Like this www.ebay.co.uk/itm/361245077596

You can also get special groundsheet pegs (flat heads).

Or look at awning carpet if it doesn't need to protect from damp grass

Thanks all for the tips. I'll go for a huge ground sheet under the tent, peg it down and put picnic blanket on top. Fingers crossed!