To begin, it must be understood that as Vanny said, working on car's is an awful experience. So by far the best method is to employ a garage to do all the work to your daily hack. However, if you are tight, or broke, or both, then DIY bodging is often the only way.
Working under any vehicle brings with it certain risks.
However any hydraulically suspended Citroën's carry a slightley larger risk.
Now, I am far from the best person to be giving advice here, since I do everything the HSE would be getting high blood pressure about, but since I have a death wish I dont really care, and I'm living proof that even with no common sense you can work on a Citroën without being killed!
So, on to business:
There are various methods employed for the purpose of lifting cars.
By far the best and safest are one, two or four post lifts.
post lifts
Since few of us have access to these almost mythical things, we are reduced to using jacks, axle stands, car ramps, bit's of wood etc.
Jacks
axle stands
ramps
bits of wood
It goes without saying that unless in an emergency (changing a tyre or the such) any car you are attempting to jack up should be on level SOLID ground.
In the case of the BX, jacking the front of the car is pretty simple, either place the jack pad under the subframe on the outer edges where the wishbone attaches, or at the very back of the subframe where it is bolted to the floor of the car, OR directly on the floor of the car just behind where the subframe bolts to the floor where the two little rubber grommits are.
Jacking the back end up is a little more difficult and requires a touch more thought since there isn't much at the back end substantial enough to place a jack under.
The best places are either the very end of the subframe where the BIG rubber bush is located, or under the radius arm where it points towards the floor, the same face the suspension cylinder pushrod is attached to.
I dont recommend jacking a BX on the sills, unless they are totally rot free.
In terms of placing axle stands, at the front end any of the three points listed for jacking, or under the strengthened part of the sill (again, if the sill is in good condition).
At the back end, the easiest place is under the sill (again where the strengthening plate is) or under the axle tube inboard of the rear arms.
If using ramps, ever noticed that they either lift or fly out as you drive on to them?
Get yourself two long lengths of carpet or the such. wrap one end right around the ramp and secure the carpet to the ramp. Then trail the carpet away from the ramp so that (assuming you are driving the front wheels on to the ramps) the rear wheels are sat on the other end of the carpet BEFORE the front wheels touch the ramps. Then the weight of the car applied to the carpet at the back end prevents the ramps from flying away as you drive on to them!
At all times, be sure that there is enough room under the car for you to comfortably fit, even if the car is on LOW.
NEVER rely on the suspension alone to keep a safe working height, they can drop VERY quickly.
In terms of equipment, always buy the best you can afford.
Trolley jacks are preferable to bottle jacks as they are more stable.
Also trolley jack quality varies enormousley, the best to get is a jack with a capacity far greater than required with wide spaced wheels. Again, this gives greater stability.
The same applies to axle stands. The wider the base, and the larger the area of the feet the better, this spreads the load and again, makes things more stable.
The back end of a BX is fairly lightweight but can still kill you if you it falls on you.
NEVER TAKE ANY CHANCES.
In this case, do as I say, not as I do....