Best for anal sex, since they tend to last longer and feel slipperier against the skin. That will help bridge the size gap between a finger and a penis or strap-on. Enough submissions with the same name. You can find lubes specifically made for anal sex. The method for insertion (she can do this herself, or you can do this in coordination with her as she signals to you) is to press the plug slightly into her ass until she feels the tiniest pain, and as soon as she does, back off slightly.
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Tight ass first time anal. Lube up the smaller buttplug. Is an excellent choice. If she enjoys dirty/sexy talk , tell her what she likes to hear as you move up her body. The vibration gives you more stimulation, and the added pleasure will help you relax and enjoy the experience more. The next day, you do it before bedtime with the larger plug. I find the hardest part for the recipient is when the head of the penis passes the sphincter. Then add more lube.
This is also very very important. She should relax and feel them, and they will subside.