1) Fit wheel and see how much flaring has to be done.
2) Remove wheel and proceed to remove inner guard/plastic splash guard. Also, good time to take a photo for a "before" shot if you are keeping a record of it.
3) Using hammer and dolly, flatten under the lip that normally has the inner guard screwed to it. This is to make it easier for the flaring part, and also means there is no sharp edge to the flare/guard.
4) Use hammer and dolly again to flatten out the roll of the guard and make it continue to the same shape of the original factory flare. Take a bit of time, but the slower you do it, the better the result. Take it out to roughly where you need it then refit wheel and see if it is enough. If more is need simply work out it further until enough room is gained.
5) Once shape is done, continue to work out the high and low spots and make it as smooth as can be had. Then use grinder/sander to remove old paint to prep it for painting. Give it a small swipe with bog then sand it all to shape to get out any imperfections in the metal work.
6) Prime and paint then stand back and admire.
And, If you get stuck or need a little guidance, recommend finding one of these:
Old man is the best helper ever!