If your company IPOs they will withhold shares for you. Only thing to watch for is depending on your income they might underwithold. There isn’t much more to it than that.
Even if you have RSUs (where taxes are customarily withheld), the company will withhold at the minimum statutory rates, which may not match up with your personal tax bracket.
If a company hasn't IPO'ed, an 83(b) election may also make sense.
After whatever lockup you have some fraction of shares remaining. When you sell those and if the price is greater than the IPO price then you have capital gains on whatever shares you still hold.
You still owe tax from the IPO. My point is the company will withhold shares from you, but you might need to pay more if they underwitheld for the IPO. This has nothing to do with your capital gains from selling your remaining shares.
All said, the better companies offer partial recourse loans to early exercise.
This is days later, so not sure if you'll see this, but could you explain what QSBS has to do with it?
If you've been at a private company for a while you may have options that have vested approaching the ten year expiration with a large spread between strike and the fair market value.
If these options are ISOs then exercise has a lot of tax consequences to get right (AMT particularly to save some money). If you have NSOs you've still got a lot of tax to deal with on exercise, but you don't have to benefit of getting some tax free below AMT.