Bodywork can be really fun or it can make you cry.
There are some general rules to adhere to.
Rule 1 - Don't use anything over 400 grit on the plastic bodywork like the front bumper. It will scar it up.
Rule 2 - Put the power tools away when you are dealing with the flexible plastic, unless it's a sander (not a grinder) with 400 grit or finer SP.
Rule 3 - Don't put filler/putty on plastic. It's flexible and will eventually break free, bubble and look bad as it peels away from the plastic.
Rule 4 - 400 grit or less.
Some will argue with my rules, so be it.
Sand away the badness on the bumper (based upon the image) and get it to the plastic.
Get a paint mask. Protect yourself. The cartridge kind are the best. 3M makes good ones.
Use wet/dry sandpaper. The black kind. When your sanding, use water and the sandpaper. Water will carry away the waste.
Do a coat of primer. If you have scarring in the plastic you can use little tiny bits of putty but I've had putty fail me on plastic too many times. If you can prime/sand prime/sand prime/sand to fill, I've had much better results. I've learned though failure.
....and then do another coat of primer. Get it to be the way you like it. Sanded nice, no finger oil, no contaminates. Nice and clean. Use mineral spirits or acetone to wipe it down. (open air, fans, etc...while doing this).
Depending on the brand of paint you use you will hand sand the primer to either a 400 to 600 grit. The manufacturer of the paint should have a white paper detailing their requirements.
Once you've primered your very clean surface that is no more course than 400 grit you can lay your paint.
Layer paint is easier if you are going to follow the paint up with a clear coat. The clear coat allows you to overspray and compensate for your failures. Part of painting well is hiding your mistakes. Clear coat allows you to fail during the paint phase and compensate for all of the mistakes you make.
When you clear coat everything becomes shiny. If you clear coat a moderate amount, 3 to 4 sprays, you get to sand it down.
First using 600 then moving on to 1000 grit. You don't have to worry about the paint if you have excellent priming and sanding. Patience and cleanliness yield the best results. After paint, you get to sand the clear coat to 1000 grit and rub out anything you don't like.
Keep it clean, keep it flat. I can elaborate on any points. Best of luck.