One of the most famous permanent portfolios is the Ivy (League) Portfolio. You can read all about it in the book The Ivy Portfolio: How to Invest Like the Top Endowments and Avoid Bear Markets, by Eric W. Richardson and Mebane T. Faber (link).
The basic idea is to keep a fixed portfolio and rebalance it every once in a while. The recommended percentage of asset classes for a European investor could be something similar to the following:
20% European stocks
20% World stocks (not European)
20% REIT
20% European intermediate bonds
20% Commodities
If you are interested, we are going to propose specific ETFs (Exchange Traded Funds) to build it. For instance, for the bonds, a possible ETF could be iShares EUR Corp Bond ex-Financials 1-5 yr, which avoids banks and has a very decent diversification. For the commodites, a very popular ETF is Invesco DB Commodity Index Tracking Fund (ticker DBC). The other 3 will be tweeted during the week.
Buying ETFs is very simple, just find the ticker and buy it like shares from your broker.