Everyone here has pretty much has laid out all the steps but one tip I found to really help out is to fill the tank with sand. It will give you sims resistance so the front section of your dents won't collapse your tank.
Man I sure hope you washed and washed and washed and washed, then watched your filter like crazy for those pieces that you did miss... It sure is a good idea, but what about filling it with water instead? Then you just have to let it evaporate? Sand just gets EVERYWHERE and it's very tough to get it all out leading to clog a filter and cause a lot of headache...
+1.... filling your tank with sand doesn't sound like a good idea to me and it isn't really necessary anyway,
putting knee dents in a tank is really easy, the hardest part for me was getting up the courage to take the first swing, after that everything just kind of fell into place,
you will be really surprised how easy it is, i used a medium sized dead blow rubber mallet and started with really light hits until you feel the metal start to move, you get used to how hard to hit real quick, just go slow.
i started on one side until it was roughly the shape that i wanted and then worked on the opposite side until they were somewhat symmetrical then just fine tuned the shape from side to side, it takes awhile but its really fun.
the thing i dont like about the method were you would cut out the side of the tank and then weld it back in is that it seems like you would have a really sharp transition going into the dent, i like the nice round edges that the pounding method gives you