If they are indeed 6 weeks old, then they shouldn't be bottle feeding any longer
Just leave them a tray with some of the kitten formula in it (I use a cookie baking sheet) Mix the formula well (I use a blender) pour it out into the cookie sheet and show them the treat. They will be lapping and playing in it almost immediately. Sometimes, you have to put a dab of the mixture on their noses to get them started.
You can't leave the formula out long- 20 minutes and it will start to air-sour. It generally doesn't last that long anyway! lol
Bless you for attempting this challenging job of raising kittens who were likely ill. It is exhausting to get up every 2-4 hours round the clock to be sure the kittens are fed, stimulated, warmed, comforted. We make such poor substitutes for a good Queen's care.