How do you make a plan? week?month or year ?
There is no specific interval for buying bitcoin. Going all-in is definitely not a good idea because of the price fluctuations. But if you can track the market closely, you will see many opportunities to buy. But if you don't want to track the market closely, then streamline the buying interval so that you can score bitcoins at various price points. Prefer monthly buying at a certain date because weekly buying may not impact much on your buying price.
Also, always diversify your investment. Here I mean, invest in real-world assets along with Cryptocurrency. The best approach is to divide your monthly investment amount in to two parts and invest in both cryptos and bank or stock market assets.
While bitcoin can generate much higher income than any real world investment, but it can fetch you huge losses as well. So if you invest in other assets, it will give you support if anything goes wrong with crypto market. Remember, bitcoin is a risky venture, so try to use other assets as well to make your finance look good.