If you haven't got a professional set up, this is the way I do it.

Firstly, plan what you are wanting to do ahead of doing it
so when conditions are right, it flows smoothly.

I pick a day outside when it is very bright but cloudy.
This give a good light but no light point source (like the sun) which gives you that bright spot.

You need to also account for where you live, angle of the sun etc. You can still get some shadow areas which aren't good but these can be minimised. Play around with a gun outside.

Use a Bipod ! This is important.

I lay out a blanket or something, felt, towel or whatever
on the ground.

It is worth having some small things to keep a gun in position, ie
Small piece of wooden dowling, 1/2 inch or so to go through the trigger guard.
- Small piece of clear perspex for the same as above.
- Box of matches for under the double barrels (they tend to be small enough to use without having to edit them in Photoshop.
- Square or rectangular blocks of perspex also work.

That's all I can think of at the moment.

Of course, with digital cameras, you can play around, practice taking photos and see straight away whether
it is good or not.

Good luck.