To skip the intro peach cutscene when you start a new file, in init_level in level_update.c, change:

if (gCurrDemoInput != NULL) {
    set_mario_action(gMarioState, ACT_IDLE, 0);
} else if (gDebugLevelSelect == 0) {
    if (gMarioState->action != ACT_UNINITIALIZED) {
        if (save_file_exists(gCurrSaveFileNum - 1)) {
            set_mario_action(gMarioState, ACT_IDLE, 0);
        } else {
            set_mario_action(gMarioState, ACT_INTRO_CUTSCENE, 0);
            val4 = 1;
        }
    }
}

to

if (gCurrDemoInput != NULL) {
    set_mario_action(gMarioState, ACT_IDLE, 0);
} else if (gDebugLevelSelect == 0) {
    if (gMarioState->action != ACT_UNINITIALIZED) {
            set_mario_action(gMarioState, ACT_IDLE, 0);
    }
}

This tutorial was originally posted by “Revo” in the N64 Decompilation discord.