I guess I should post some comments because I have actually done it, it has worked for regular shoes but it didn´t do much for my heavy old Italian hiking boots (this was in 1976), I guess they were just too tough and thick.
If I remember correctly and did what I´ve done to other shoes: I fill with hot water, empty and fill again, empty and put on lacing tight.
If you can exchange the boots for a better fitting pair that´s what I would do, what is usually recommended is to try them on in the afternoon or evening when your foot has swollen a bit and use the same socks that you will normally wear with them.
If you are stuck with this pair you probably won´t loose much by trying, something to think about is if they have Gore Tex or some other modern insulation which may not let the water work, also consider that they will likely take a long time to dry.
Luis