Whatever amount of money you're willing to allocate for anything you can split it up into 1/2s, 1/3s, 1/4s, 1/5s or what have you. You don't have to buy a position all at once, nor sell a position all at once.
Frankly, if I ever happen to get in on the bottom it's plain luck rather than anything else. I look at charts, technicals, patterns etc.. but still there's no way to know fo sure where the bottom will eventually be or the top for that matter. Part of why it's probably prudent not to shoot your shot all at once. Retail level don't even have to pay commissions anymore like the old days either. So you can split it up as much or as little as you like.
Honestly, I probably end up doing better when I don't pick the absolute bottom because I'll have ended up accumulating more shares than I would have if I did. Main thing is I usually only start buying after there's a nice cushion/drop first and I use charts to help with potential buying (and selling areas).