1. Download the latest acsutils-x.x.x.zip.
2. Copy acsutils.acs into your project.
3. Include acsutils.acs after zcommon.acs:

#include "zcommon.acs"
#include "acsutils.acs"

If your project has functions (or even variables, if you are using ACC) with the same names as ACSUtils functions, it won't compile. Rename the functions in your project to fix it. ACSUtils functions are named to avoid some of the most annoying conflicts, but several renames are still needed in big projects.

If your project has functions that are already present in ACSUtils, you can just remove them.

1. Copy decorate/changeflag.txt to your decorate and #include it to make ChangeFlag work.
2. Copy cvarinfo.acsutils to your project root to allow the server to hide ACSUtils errors using the acsutils_noerrors cvar (it will automatically load and not conflict with your mod's CVARINFO because ZDoom ignores the extension).

If you are using BCC, try BCSUtils – a BCC-enhanced strictly typed version of ACSUtils with the same functions!