This is what Pop always used, and when I inject its what I use now.
2 oz. bottle Liquid Onion
2 oz. bottle Liquid Garlic
1 small bottle tabasco
1 small bottle Lea & Perrins
1 stick butter
1/2 cup salt
Remember to heat it and dissolve the salt then let it cool, undissolved salt plugs the needle. If the poultry is cold, the butter can clot in the needle. If you want to add herbs and spices, steep them as you would do a pot of tea, then strain off the solids.
Us it sparingly, try to get it in the muscles. Inject numerous time with just a little to prevent pooling. Extra keeps well in the reefer for a long time.
Like everything else in smoking, use a light hand on your first attempt, the next attempt you can adjust.
Hope it helps you.
There are numerous brands of premade injection on the market. Most have too much salt. Buddy used a butterball injects Cajun flavored.
Bon Chance