It depends on the type of ivermectin.
Injectable is giving individually orally. Drop on (my personal preference) is given via drops between the shoulders.
If you give injectable, it's done best if you also give it with glycerin. Otherwise it can pass through the system at such a rate that it doesn't treat. But you can just do it normally, too.
But there are two very important bits of information you need to know before touching the stuff.
First, ivermectin kills all worms - strongly and quickly. If you don't worm twice a year, then I would worm first with something like Wazine and dust your birds and premises at that time. THEN go back in 14 days and treat with ivermectin.
The reason is that if your birds have a heavy infestation of worms and you don't know it, killing them all at once might cause them to go into shock and/or have blockage of their digestive tract (upper or lower) from dead worms. So you use a lighter repeat-dose wormer first (piperazine 17% aka Wazine 17) to do a lighter worming of just roundworms.
Because you don't want to wait to get on the mites, use a poultry dust (permethrin for example) on the birds all over their body. Also treat their bedding (scrape it aside, lay dust down, replace it - dust the top of the bedding, stir it in), nest boxes, etc. That will knock down mites and also feather lice.
Then in 14 days use the ivermectin of your choice. That will kill the remaining roundworms and all other normal worms except tapes. It will also kill the remaining mites that hatched out, lice, etc, and is said to have a 14-28 day residual effect.
Then just worm with ivermectin yearly to twice yearly thereafter. Spot check every month for mites/lice on your birds; dust premises every time you change bedding. Treat as needed with ivermectin when wild birds bring mites in. You can try using DE in the dust bathing areas as a helpmeet in controlling excessive parasites.
So in summary:
Step one:
Unless you worm 2x's a year, assume your birds are wormy and worm lightly (wazine) first.
Treat the bedding/premises at the same time.
Step two:
In 14 days, reworm (and treat for mites/lice) with ivermectin.
Step three:
Repeat step two twice yearly or as needed if wild birds bring mites in.
Please feel free to email me with any questions on the above, or anything else.
Added later: Duh. I forgot to include this for you:
https://www.backyardchickens.com/forum/viewtopic.php?id=208346