It's important to give offering to any spirt you work with, think of it more as a thank you gift then a pay check. This also shows a welcoming hand in friendship or a simple acknowledgment that you know, they are around. You must remember that most spirt only take the energy from the food, so don't be disappointed if you find the food still in the place you left it.
You can leave your offering either inside or outside, but make sure to remove them afterwards. With food offering I normally remove them the following morning, but I have heard people remove the food offering, after three days. You can burrow it in the ground, or in the garden afterwards. This way it goes back to Mother Nature and it's like the cycle is continuing. If working with say a God or Goddess then it may be wise to look up what kind of offering they prefer, as like people spirt also have their own likes and dislikes.
Bread (fresh, can be home made.)
Fruit (make sure it's always fresh.)
Whine or alcoholic
Unless it's an ancestors who you know how they had liked their coffee, you can just leave them black coffee, no milk or sugar just black. You can also leave them meals they enjoyed in life.
When it comes to cook meals, try to give them the first serving of the dish and not the left overs. You also should be mindful about the types of offering you leave for the spirt. If you work with a war God, then it might not be a good idea to leave them flowers.
Bread is also another great offering, make sure it's fresh and if possible home made is normally best. However, the fresh bread from the baker is best thing since not many people make home made bread anymore. Again first helping, fresh fruit as will. You can slice it up if you feel that it's the best thing or you can leave it whole. You can place them on a spare plate.
Drumming is a very earthly sound, and like us there are some spirit who like a good beat. If you can't play the drums yourself then you can play some music from YouTube with the intention that it's the offering.
If you can write poems then there's another good one, you can burn the paper afterwards and picture the smoke taking the words away to the spirt in question. Please remember to use common sense for this, and be mindful of where, when and how you use the candle during this process and stay safe. Fire hurts.
Incense you can burn it in that spirt name as a offering, you may feel the need to buy a certain one as they pull you to the one they prefer.
Thank you. While most spirt may not like the word thank you or even the offering. Angels are ones, who are normally happy with just the word thank you. Again after you work with a spirt, you may feel drawn to a certain offering, then I suggest offer it to them as it would be something they appreciate.
It depends on you and your work. If you are drawn to then you can leave an offering at the start of the month and at the end of the month. It may also depend on how often you work with that said spirt. If say you work with a spirt who deals with winter, you could probably leave an offering for them during winter solace.
This is just some basic knowledge about offerings, I recommend to always do your own research.