i just did this this past weekend with mine, does not make me an expert however, this was my process.
i bought a rat pup had no fur at all on it. he was not having it at all, same thing he would just spit it out. so i got a mouse pinky, it was only 1 gram, i put that in his mouth and gently pushed, held it for a minute or so and then bam, he started taking it down. i placed him back in the tub and let him be for a little while, me knowing that 1 gram is not enough food for him, i tried again with the 5 gram rat pup, it was the same one that i tried the first time, i ran some warm water over it and then dried it off, then the same process open mouth and gently push. once i felt it was in far enough i waited for a min or so and then he took that down too! i am hoping that i wont have to do it anymore, i hope that next week when i try again that they will take it on there own.
i wish you luck, there are videos on line, i am pretty sure i watched all of them. just watch some to build your confidence and give it a go.