Give offers after the demon has fullfilled their emd of the bargin. Its a business transactions theyre providing a service in return you give them X. You dont pay them upfront otherwise they might just run off with you offering because you’re being a chump.
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Its importnant to use your imagination to offer the spirit some energy to manifest and speak through.
Make sure to banish before the evocation as well.
You can ask for a sign the demon is there like beelzebub touch my arm if you are here. Or something similar. Candles could help to. Once you evoke him the candles will probably fluteer or they might even go out.