I would say make a prayer to your deity or deities whenever you feel that you need their help or their strength. Making a prayer to Loki can be the same as if you made a prayer to the Christian God, or at least you can make it the same sort of way.
In a prayer, you say the name of the deity/deities you are looking to get aid from, then you tell them what's happening and what you want of the situation, then you close with some sort of closing statement. In my case, when I do not have the ability or time for an elaborate prayer, I tend to go, "___ please help me with ___ . Thank you." It doesn't have to be complex, nor do you have to say the same prayer over and over. If you are working with a deity that enjoys offerings, offer to give them something in exchange for their help, or offer to build a small area of your altar to them if it is something bigger. If they are your main deity/deities, you can offer to give them something special just for them.
When it comes to the deities, it's really just between you and them what you do.