Normally asset allocation is done through Modern Portfolio Theory. But value investors such as Warren Buffett $BRK.B do not believe in Modern Portfolio Theory.
Asset allocation is done through a top-down approach. First, the investor is going look at macroeconomic factors to see in which country or asset classes, he/she should invest in. For example, if he/she expects higher inflation, he/she will invest less in equity $SPX500 and more in gold $GOLD .
Warren Buffett has a Bottom-Up approach to investing. He looks at the company first. If is is a good company and the stock is undervalued, he's going to invest in it.
For a Value investor, asset allocation is not really about looking at macroecomics and to determine how much of your portfolio should be in bonds or stocks but rather at investing in what is cheap. If stocks are cheap, invest in stocks. If bonds are cheap, invest in bonds. Warren Buffett Invested in Apple $AAPL not because he wanted exposure to US tech but because it was undervalued.
Today, however, with the amount of information available, it is possible to have a top-down approach and be a value investor. For example, you can look at which sectors are going to do well because of a certain economic or market condition and look for companies in that sector. I invested in Skyworks Solutions $SWKS because I was looking for 5G stocks. I was looking for copper $Copper miners first before investing in Freeport-McMoran $FCX .