First consult you BMW hand book to find the front and rear track widths, many cars have a different front to rear track widths and this dimension has to be allowed for when setting if the reference lines parallel to the side of the car.
You will also need to know, the toe in measurement for the front and rear wheels in minutes, degrees or 'mm', depending on your source of information.
Spread sheet attached to save monotonous calculations
Setting up the reference lines to measure from:-
As i have a 18 year old BMW E36 with numerous scuffs on the bumpers, i opted to screw the brackets directly to the bumpers as a few small holes are not going to detract from its market value of degrade the overall appearance. If you have a pristine model or painted bumpers i would recommend sourcing suction cups and attaching the brackets to those so not to mark the bumpers.
Alternative methods are to use 4 axle stands with the reference line tied direct or source 2 x 2000mm long poles, drill some holes 50mm from each end, place these on the axle stands and string between the holes for and aft.
The only problem using a reference line not mounted to the car is that when rolling the car back and forth to settle the suspension, you may have to check the alignment of the reference line again and again before measuring to check your progress.
Things to note, the distance from the reference line to the wheels is not important, the line must be parallel to the car on each side, as you are measuring a difference between the front and rear edges of the wheel rims. For ease of measuring i would recommend using a steel rule and the reference line to be 30-50mm from the rims and the line level with the centre of the wheel hubs.
here is a picture of my line mounting brackets
When setting up the reference line, measure from the centre of each hub to the line, adding the difference in the wheel track width to the front measurement, on my 316i, the difference is 13mm (1431-1418=13mm) i.e. 6.5mm each side at the front, see picture below.
Next jack or ramp the front of the car up to gain access to release the track rod locking nuts and free off the locking collars, these are likely to be seized, not explaining how to un-seize them here.
Pic below of the RH track rod components looking from the front.
Lower the front of the car, release the steering lock and centre the steering wheel, wind down both front windows and place a plank of wood across the doors and up against the steering wheel (i used my plank for reference for the steering wheel centering ,(see pic). Then clamp the steering, this is important as when you adjust one track rod end, it will move both front wheels and the steering wheel, and you will be tearing your hair out trying to keep centering the steering and measuring stuff, clamping means you can concentrate on just the measuring of each side.
Ok ball park figures here, Yank BMW M3 owners forum guru's recommend 0.2' degrees total toe in at the rear (thats 1.5mm each side) and 0' degrees to 0.2 degrees at the front, thats (0mm to 1.5mm each side) for MY RIM SIZE, ON MY CAR, please check using the spread sheet provided for the measurement in mm using your rim size measurement and/or the recommend alignment for your BMW.
Ok procedure is, MEASURE, ADJUST, ROLL CAR BACK and FORTH, CHECK MEASURE. rinse and repeat.
Typically on MY CAR, dimension A is 21mm, dimension B is 19mm, A is larger than B indicating that the the front of the rim is turned in, i.e. toe in.
Note 1 turn of the track rod adjuster will equal around 3mm of wheel alignment change, go slowly in 1/3 or 1/2 turns.
Remember after each adjustment roll the car back and forth a few feet to settle the suspension.
When you are happy, jack up the front and do up the locking nuts, hold the adjuster while doing up the locking nut as not do disturb you adjustments.
Rear is the same procedure, only a bit of brute force is required, loosen the console retaining bolts 1-2 turns, grab the rear trailing arm and push/yank to get gross movement, bang tire with the back of your hand for fine adjustments, remember to roll between adjustments. When doing up the console bolts, be careful as the rotating motion of the bolts throws your careful alignment out.
Take you motor for a 5 minute drive to settle everything then string up and check everything, adjust again as necessary.
Note: if you Front or rear TCAB's or your steering rack or track rods ends are shot you are wasting your time.
When driving forward/breaking the natural tendency is for the front to toe out under-load, hence all cars need some toe in to counter this.
My car in particular feels like I'm turing right slightly all the time with the the alignment spot on due to the camber in UK roads, i normally add 1mm of additional toe in at the front right track rod to counter this (front right tracks straight, left front left more toe in to climb up the camber), i may have a worn out track rod as the front is powerflexed.
I take no responsibility for errors, worn out tires or accidents, you can of course pay a garage or a stealer for doing this for you, but there's no reason why a competent home mechanic cant do this.