To prep a scion you remove its leaves and leave it on the tree until its buds start to swell, so you know it's full of energy and ready to push. My scion was from a HUGE seedling sap with spectacular fruit we want to propagate. Especially because the tree is so large, it took several weeks for even the slightest movement of the buds. I cut it as soon as the tip bud started to green up a little.
I'm NOT an expert grafter. I had a miserable take rate of about 50% with avocado and citrus (the easiest to graft) and 0% with mango. I got my 50% with clefts and modified clefts - I don't think I ever did a successful veneer graft. My luck changed when I started doing a version of the "gravedigger" graft (search the forum for gravedigger and you'll find the thread), which I would say is a modified bark graft. Suddenly my citrus and avocado percentage shot up to over 90% and I have similar results with mango. I cut off the branch where I want to graft, peel down the bark as for the gravedigger method (usually I leave a flap but it's not necessary) and cut the scion with a long cut on one side and a short wedge cut to a point on the other. I don't have buddytape or parafilm so I use regular plastic wrap which I like because I can pull it very tight for good contact, but I have to be careful to remove it after the graft takes or it will girdle. I'm mostly topworking established trees/seedlings and this method has been virtually fail-proof doing that. Rootstocks in bags are harder to graft because they just don't have the vigor of planted out trees with good root systems, so my luck with those has been considerably less but still better than I had with other techniques. The only caveat with this method is the rootstock needs to be bigger than the scion so the flat cut of the scion has a flat surface to adhere to, otherwise you can only get contact on one edge and it's much less likely to take. If you're topworking or working on established seedlings it's easy to have rootstocks thick enough for most scions, but that can be a problem for rootstocks in bags. If you can, I recommend planting out rootstocks and letting them settle in for a year before grafting. Grafts are more likely to be successful and they take off like rockets on established trees.
As for the latex, I pretty much just ignored it but I do think the first attempt might have failed because I went too slow and maybe the latex started to dry up a bit. The second time I worked more quickly. I also did one with a non-prepped scion that looked it had a swelling tip and it hasn't started to push yet but it's still green. Like I said, you're going to wait either way.
Hope that was somehow helpful!