OS: Debian 10
Desktop Environment: XFCE

Install RDP on a system running a desktop environment is really pretty simple with one caveat. It seems that you cannot steal a session from the console by logging with that same username. So, if you know you are going to need the desktop environment, make sure you log off the console, if logged into the desktop environment, before attempting to login remotely. If you do forget, you make only get part of the environment, but have no way really do anything until you logoff from the console session.

Install RDP:

# apt install xrdp xterm

Enable xrdp and xrdp-sesman:

# systemctl enable xrdp
# systemctl enable xrdp-sesman

Start both services:

# systemctl start xrdp
# systemctl start xrdp-sesman

If you do not have a desktop environment setup:

# apt install xfce4 xfce4-goodies xorg dbus-x11 x11-xserver-utils

Verify that xrdp is running:

# systemctl status xrdp
● xrdp.service – xrdp daemon
Loaded: loaded (/lib/systemd/system/xrdp.service; enabled; vendor preset: enabled)
Active: active (running) since Sun 2021-03-28 19:49:01 PDT; 4 weeks 0 days ago
Docs: man:xrdp(8)
man:xrdp.ini(5)
Main PID: 724 (xrdp)
Tasks: 2 (limit: 4915)
Memory: 22.2M
CGroup: /system.slice/xrdp.service
├─ 724 /usr/sbin/xrdp
└─19572 /usr/sbin/xrdp

Apr 26 10:22:30 lm xrdp[19572]: (19572)(140553222580032)[DEBUG] xrdp_wm_log_msg: connecting to sesman ip 127.0.0.1 port 3350
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[INFO ] xrdp_wm_log_msg: sesman connect ok
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[DEBUG] xrdp_wm_log_msg: sending login info to session manager, please wait…
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[DEBUG] return value from xrdp_mm_connect 0
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[INFO ] xrdp_wm_log_msg: login successful for display 10
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[DEBUG] xrdp_wm_log_msg: started connecting
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[INFO ] lib_mod_log_peer: xrdp_pid=19572 connected to X11rdp_pid=19575 X11rdp_uid=1000 X11rdp_gid=1000 client_i
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[DEBUG] xrdp_wm_log_msg: connected ok
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[DEBUG] xrdp_mm_connect_chansrv: chansrv connect successful
Apr 26 10:22:31 lm xrdp[19572]: (19572)(140553222580032)[DEBUG] Closed socket 18 (AF_INET6 ::1 port 59468)

Add xrdp to ssl-cert group:

# usermod -a -G xrdp ssl-cert

Reboot:

# shutdown -r now