Frog Stand To Handstand

I need help on my frog stand to handstand I have problems pressing up into the actual handstand I can’t seem to keep my back straight and just pressing proves to be difficult for me any advice would be greatly appreciated so I can apply it in my hand balancing session tomorrow 

Likely you just don't have the strength.  Are you aiming to keep your arms straight the whole time?  Because a press to handstand is harder than the frog stand->headstand->handstand.  If it is the first then you should do slow negatives from handstand to the frogstand while keeping straight arms.  If it is is the latter then you will need develop the headstand pushup strength necessary then practice getting to headstand from frogstand.  

Hard to tell without a video.