How difficult is it to replace the struts on a 1999 Corolla?

It depends on the tools you have, and the environment you will be working in. The best thing o do is get a book from the parts store on changing struts, or go read about them at a Public Library. The job itself is fairly easy.

I did it last year, the rears were not too bad but the fronts, the lower control arms don't drop far enough without disconnecting the axel shaft and tie rod. But it can be done, be prepared to muscle them in place. That is one job I will never attempt on a Toyota corolla again. On the other hand I did it on a 1987 for Thunderbird, very easy. It is a car model by car model decision.