Seat Belts: If your manual wheelchair comes with a seat belt it may already be attached if not you will need to unscrew two screws on either side of the back and loop in the seat belt.
Wheelchair cushions sit on-top of your wheelchair seat - if you find it slipping off you can add velcro to the bottom of your cushion and the top of your wheelchair seat. Some cushions come with a “non-slip” material on the bottom.
If you are a little bit handy and have some strength, you should be able to drop in the batteries, connect the cables and place the seat on top of the wheelchair base.
Also consider: what side should the joy stick be installed on?
If you're right-handed, it should be installed on the right. And, if you're left-handed, it should be installed on the left. Most power chairs, if the joystick is installed at all, are delivered with the joystick on the right side. If you need it on the left, mention it to your salesperson when purchasing or you can switch sides when you chair gets delivered.