If you will be presoaking seeds like I do (for 24 hours) I suggest starting a week before plant out time.

In my experience presoaked seeds germinate in 1-3 days and grow rapidly after that (I use a heat mat).

If yours usually take longer to germinate then start earlier, but I have not found any value in planting foot long MG. They seem to self terminate the leader for awhile. Soon as they sprout and put out a couple leaves it's time to plant them in my opinion.

Otherwise you can direct sow them as long as the ground is fairly warm and if presoaked they will also germinate quickly.

I have morning glories all over the place here. I presoaked some once and direct sowed outside next to my antenna tower a few years ago. They grew past our second story roof! They've been reseeding themselves every year and are a riot of color all summer. They grow really fast so if I were you, I wouldn't bother starting them indoors. Just a thought.
Karen