Here are the simplest, but not the weakest, ways to use our funeral gifts and offerings, but you are free to use them in your way and tradition, with our without prayers (spells) and anything else.
As a funeral gift or offering for the deceased and powers: put herbs (or the oil) on the grave or on/ into the coffin after or during the funeral ceremony with a clear intent (your wish or blessing for the deceased).
In the same way, plants are used for the protection from the undead and restless souls, and for the curse of the deceased and bounding them. Folklore keeps memories of this practice with humans and with supernatural beings, like stabbing a vampire with a wooden stake to keep them chained and out from the human realm. Modern religious funeral ceremonies and prayers mainly focus on purifying the space and banishing the "evil".
As a remembrance gift: put a herbal bag on the grave or a few drops of the oil on the grave (and near, if you want) with your clear wishes to the deceased and concentration on him/ her. Tell him/ her what your gift should do to him and anything you want, but be specific. You can do the same at home near his/her photo or something related to him/her (like his/ her favorite ring, toy, clothes and others).
Also, you can inscribe the name of the deceased on the candle, after that dress it with oil and light it. Focus on the departed.
Do it once or as a ritual for 3/ 7/ 9/ 40 days on the specific dates like his/her birthday, date of the death, after the death (or the funeral ceremony), wheel of the year, esp.on the Samhain (October 31st / November 1st) and on the Yole (December24th/ December 25th) or every time you feel the connection with the deceased or need his/her help.
For ancestor worship: keep herbs or herbal oil on the altar or use in your rituals with respect to your tradition. How your altar should look like?
For spiritism: put a few drops of the oil on yourself (read more here) or on the candle with the inscription of the spirit's name or signs. Also, you can use our herbal bags by keeping them on the ritual's altar and focusing on your wish but only once for the specific spirit.
Look for signs. There are no coincidences. Everything has meaning, but keep in mind that the nature speeks the other language.