EC help needed

So here's the situation... DS isn't ready for toilet training yet (he's 17 months give or take) but with all the hot weather we've been having I've been giving him a lot of nappy-free time when we're at home. It occurred to me (after cleaning up puddle after puddle and the occasional number 2 ) that i'm wasting an opportunity here where I could be teaching him about EC in preparation for TT later on.

Just wondering how I go about starting it?? I've been watching for signs and have figured out when he's going to poo (that's obvious enough) but always miss the fact he's going to pee until it's too late.

We have a potty - just a basic cheapy from the warehouse that I have tried to get him to sit on occassionally with no joy. Is this okay, or do we need something nicer? and is it better to have the potty in one particular place all the time (ie the bathroom) or move it around the house?

sorry for all the questions, but i've never really looked into this before! I've done a bit of internet research but would love to hear people's personal experiences - and tips (please?)!

We had a warehouse potty too, and when i made taine sitr on it, I found marks on his butt. they are quite uncomfortable it seems. I would've bought him another one with wider seat but he decided he would just use the toilet instead,

not really, pants are more breathable than nappies. and not as tight fiting. so if the reason for napy off time is fresh air then he is still getting some.