Our chickens are friendly, so you can just reach down and pick them up. Since I'm married, my husband and I can do it together. One can hold the chicken while the other clips. If you're doing it yourself, picking them off the roost would be an easy way to do it.
We do it when they need it. It varies by season and by chicken. They free range during the growing season. I notice the ones that might need it, need it more after a long winter. Some never need it. Sometimes it's just certain toes on certain chickens. We also have a breed with a 5th toe, so those need to be trimmed.
I like to give the chickens a treat on the patio every day. It's a great place for me to check their feet, as I can see them really well on the concrete. I check nail length, look for any sign of scaly leg mites or swelling. I also look at their comb, wattles, eyes and feathering. Spring is when I usually do a nail clip on any that need it.
On a day when I want to clip nails, I just bring the treats, the dog nail clippers and my husband along to the patio.
Nails have a vein with blood in it, in the center. It's what nourishes the nail's tissue and allows it to grow. As the nail grows, the vein recedes, so the end is just dead nail. Closer to the end of the toe is the vein. On a light colored nail, you can see this vein or quick, as it's called. It will look pink. You don't want to clip this part or it will bleed. Just stay a little farther away from the pink part.
You definitely want to trim the nails if they are too long and causing problems with the toes. I think once they get too long, they don't wear down as well, either, if they don't have a normal shape and position. Then I think it just gets worse, faster.
