Preseed question!

After hearing so many great things about preseed I want to go buy it!

We typically never use lube but if we do we use Vaseline (is that bad for ttc?)

But I'm just curious (maybe is says this on the box but I haven't gotten it yet lol) when are you supposed to insert it? Like before you start fooling around? Or do you insert it right before sex...? Just trying to figure this out because hubby & I tend to foreplay for a while before so is it okay to put in before play time or does it have to be right before sex?