I'll give you the kindest advice, be prepared to have some unrealized loss in the beginning. Go for averaging out, suppose you have 10k to invest. Just go with $2500 on your first buy. Then wait for market to go down if it does buy another $2500. Remember the point is to buy in the bearish times and not in the bullish times.
The best advice, buy with several stages. This I also do in trading. But the first piece of advice I give is to choose the best altcoins. The best here may be a little difficult, but for sure there are some coins that have been able to survive to date and thrive instead of the hype. Maybe if it's Bitcoin's choice, right now maybe the price is still quite cheap if you look at the ATH that has ever happened.
Indeed, no one knows when the price will fall, but a trader must know that there is an analysis that must be used. Technical analysis that many traders already know and agree on. Well at least that can help,although in the end the market may change because of the owners of large funds(whales).