PS It's hard to do this "mobile", but it makes a difference if you mean in a vehicle (correct use of "mobile" in ham terms) or on foot ("pedestrian" in ham terms).

The hardest part is the antenna.

If you want a small antenna, so you can walk around and then set up the radio wherever you stop, then a reel of wire in your pocket will do, but then you need to string it up from a tree or something (which takes time to set up and take down).

If you want to talk while walking you could build an HF Man-Pack, which is a backpack radio set up with a tall whip antenna, but it could be bulky, and the whip is quite long.

On a vehicle, an HF whip is still long, but it can be left permanently installed.

Everything is a compromise, but with experimentation comes experience, so you can find out what works best for you.