Follow the sane advice and dollar cost average - set aside a small amount each week, say $50, and use that to buy btc no matter what the price is. Have a plan to hold the coin for a certain amount of time say 2 years or when it reaches a certain price threshold say $20k. This way you are getting some exposure, with greatly reduced risk compared to dropping the whole lot in at once.

One thing you can do is look at historical trends and buy when the price is near the bottom of the band (which it is now):

https://www.blockchaincenter.net/bitcoin-rainbow-chart/ https://bitcoineconomics.io/beam.html

Or you can use a stock-to-flow model and buy when the price is below the model's prediction:

https://digitalik.net/btc/

Simple answer is : when the price is low. If you don't wanna wait or have to figure out the if & when, dollar cost averaging is the most basic strategy https://en.m.wikipedia.org/wiki/Dollar_cost_averaging

The website dcaBTC.com provides some useful information and a calculator/charts about dollar cost averaging bitcoin: https://dcabtc.com/