I had to do it twice and discovered something that really helped me not to fry the cement - or have cement ooze around the seal and stick to the the rim of the stove - whch will tend to pull your gasket apart when you lift he hopper.
First be sure to scrape the old seal channel well. Also make sure you put the seal in loosely - do not stretch it tight around the perimeter or it will tend to spring back out of the channel.
This next trick only works on a horizontal gasket like the hopper lid on my hitzer.
Lay a trail of ashes maybe a half inch thick and as twice as wide as your gasket on the rim of the deck where the gasket is going to make the seal. This way if some cement oozes out it wont stick to the top of the stove but will instead pick up the ash - which by the way does not burn and is an insulator. No heat or gasses will pass thru this cushioning layer of ask compressed between the gasket and the rim. Another advantage is that with this line of ash between your gasket and the hot stove rim will prevent you from frying your cement before it cures and you can burn your stove as normal instead of having to let it cure for a day on low temps. The next time you fll your hopper you can remove the layer of ash - whatever little bit stuck to the gasket is just added insulation.
My hopper gasket is perfect now.