Assuming you're using FPCUpDeluxe (free), and you have FPC and Lazarus installed through FPCUpDeluxe;
1) In FPCUpDeluxe, install the ARM compiler:
Click the tab "cross", check "aarch64" and "darwin" and click the "Install Compiler" button.
The FPC ARM compiler will now be installed next to your Intel compiler.
2) In Lazarus, you will need to compile the Intel and M1 binary independently.
In Lazarus got to the menu "Project" - "Project Options". Under "Compiler options" go to "Config and Target".
So far I found that, per platform, you only need to set the "CPU Family" to either "aarch64" (M1) or "x86_64" (Intel).
Select one and build the binary file. In the project directory now rename the file by adding the CPU family.
Do the same for the other CPU family.
Set target CPU to Intel and Build the Intel binary, and rename it to "project1_x86".
Next set the target CPU to aarch64 and build the M1 binary, and rename it to "project1_M1".
3) In Terminal merge both files to the Universal binary by using the "lipo" tool.
lipo -create -output project1 project1_M1 project1_x86
You now have a universal binary which needs to be used to replace the symbolic link in your final app bundle's MacOS directory.
You can check that you have created a universal binary with this command:
Output similar to this:
project1: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64]
project1 (for architecture x86_64): Mach-O 64-bit executable x86_64
project1 (for architecture arm64): Mach-O 64-bit executable arm64
Note that binaries for M1 must be signed, as it seems that they will not run on any M1 Mac if not signed.
Since I do not have M1 hardware, I'm unable to test the signing process (my usual signing process of the universal binary seems to work OK for Intel Mac's).