Ask around at the next trial, until then my advice is be really really really careful. Did I mention to be really careful?
They are super useful pieces of kit, but can cause a lot of injuries if you're not careful.

I did see a bit on David Bowyers web site about use of a farm jack. Try searching his site for some useful information to start with. There is nothing like experience but its a good start.
I agree with James advice, good bit of kit with careful use and think about what you are doing. Mine got me out of a rut when laning on my own

but I also had an experience with my Disco slipping sideways with no wheels on the front axle when doing some work
Fred, click this link and it will take you to the learning with David Boywer page. I agree with John. I met him once and he came across as 'I know it all and you don't' but I got what I needed that no-one else had so ............
http://g7tgr.demon.co.uk/kernel.php?sitena...81431421a742bdfKev
Rule number 1!
Don't ever, for any reason let your head or parts of your body that you'd like to keep get near the handle when lowering a vehicle. The weight of the vehicle is on that handle. You let go, and whatever is in the way (usually a head) is gonna get broken. google for "hi-lift injury" for proof. Some great looking scars, and worse out there.
Rule number 2.
They are unstable lift devices. When raising a vehicle, be very aware that it may well slide sideways. Especially on slopes, not just sideways but forward or back too. In fact, one of the recovery techniques a hi-lift/jack-all is good for is getting out of holes or ruts. Jack the affected end of the vehicle up, and then push the vehicle sideways. As the lift falls in that direction, the vehicle (hopefully) will be moved out of the hole. Great for that purpose, but falling over vehicles can really ruin your day otherwise, especially if there are only 3 wheels on at the time, or even worse, if someone has gone underneath to work on something... But none of us would be that dumb, right? If there is any instability, i often use a convenient winch cable attached from the upslope side to the vehicle to keep the hi-lift from falling.
Rule number 3.
If you are carrying a hi-lift/jack all, you will also be carrying a small can of WD or 3in1 oil (i refer to it now as "hi-lift juice"). The pins like to stick, usually when you are trying to lower a load. Then you are stuffed with a vehicle in the air, that you can't get down. I've done it. Every time i go wheeling, i give the pins a squirt of oil, and be ready to do it again if needed while out on the trail.
Rule number 4.
They sink. Mud, sand, whatever. The bloody things have a very small area to spread a large load. I usually have a flat board, or broad bladed shovel to put under the foot of the lift. Makes a world of difference.
(Obvious) Rule number 5. They are bloody heavy. If you have one inside the vehicle with you, bolt or tie it down. If you bounce around hard enough, or god forbid roll the vehicle, that jack is coming at the back of your head. Not good.
Hope this helps. Tim.