Techniques for adding yeast at bottling

Looking for tips on adding yeast at bottling. I am bottling three high OG beers for Christmas over the next several weeks. They have been in secondary for four months. I am using wyeast activator pack of the same variety that i used during primary. Do I just smack pack and add to sanitized bottling bucket first then rack beer on top of the yeast or do I add as it filling the bucket, or after it has completely filled. Do I need to stir. Do I need a yeast nutrient. When I fill bottles do I not fill as high as normal to decrease chance of geysers. What other techniques are used for this. Any help would be appreciated as I do not want to ruin a batch with this much time involved already.

You won't have a problem with gushers unless you have an infection.

Personally, I would set the wyeast packs aside and save them for another brew. They're too expensive to use them how you're thinking.

The little amount of fermentation that takes place because of the priming sugar is NOT enough to add flavors or aromas to the beer. Because of this, you can use any dry yeast right off the shelf. No need to rehydrate. Just buy an 11g pack of yeast and sprinkle 1/3 of it into each of your bottling buckets along with your priming sugar. You don't need the full pack. Rack your beer on top just like you would normally, and the yeast will mix just fine. Don't worry if you see a lot floating on the top...it's not a big deal.

For a big beer or one that's been in secondary as long as that, I'll typically add 1-2 million cells per ml of wort, which breaks down to about 1/5 of what you should use to pitch in the primary. Nothing else changes as Surthrn said, adding more yeast will not increase carbonation more than normal, just ensure there are enough around to eat the priming sugar effectively. If you go overboard, there will just be more sediment at the bottom of the bottle.