There's no "best" way to strum---fingers, thumbs, picks, and combinations can all work well. Eventually, each uke player develops her/his own style (or a few styles).
To help develop your upstrum, I'd recommend upstrumming (with no downstrums in between) softly and very slowly---like one strum at a time. Pay attention to how your fingertip makes each string sound. As these slow upstrums start to sound and feel good to you, begin to upstrum less quietly and more quickly---along with a slow foot tap. If you notice a problem, slow back down and play a little more softly. Keep increasing volume and tempo as you gain confidence.
Once the upstrum's feeling good at normal volume and tempo, integrate it with your downstrum. If problems arise, slow down and play a little more softly.
Have fun!