Installing aerials in easy, providing you follow a few basic rules, and will result in detection of rolling stock easily.
When installing a system for the first time, please follow the Out-of-Box testing advice first (press button below to go to Testing page).
Once you have installed one or two aerials, then it becomes much easier - remember extra aerials are available if required, or if you find you have placed them in the wrong place.  Attaching with masking tape to start with is probably a good idea, as this can be removed - the sticky back on the aerial is more or less permanent.

In general, you should use Large aerials for O-gauge, as these will give a larger read distance, and also pick up data at scale speeds to at least 100mph (160kph). Because of the spacing of the 0-gauge tracks compared to the aerial size, you can place aerials on adjacent tracks in a line.  If you have aerials in places where you know the speeds will be slow, such as sidings, then you can use Small aerials.

With the NMRA minimum track spacing in HO of 50mm (2"), you can place Small aerials in line across parallel tracks as shown, providing your baseboard is 13mm (1/2") or less.

With NMRA minimum track spacing of 50mm (2"), and a baseboard thickness of up to 25mm (1"), you should use Large aerials.  These will need to be staggered as shown in order to keep the read distance of 42mm.

With 67mm OO/HO track spacing, Small aerials can be mounted in line across the tracks, and there is even space for an aerial between the tracks to monitor points or crossovers.

With 67mm track spacing, and a thick baseboard, the aerials must be staggered as shown.  You can also monitor points or crossovers as in the diagram above, providing that the aerials are spaced 90mm away from any aerials on adjacent tracks.

Small aerials can be placed in line across parallel tracks as shown. If the N-gauge tracks are closer than 33mm, then the staggered system should be used, keeping the 50mm stagger distance.

Aerials can be mounted across the tracks, rather than in-line with the tracks.  A stagger system must be used, as shown.

Z-gauge aerials need to be mounted in line with the track.  If the track spacing is less than 22mm, increase the stagger distance from 45mm to 50mm.

The picture shows a dual reader with 13cm small aerials, spaced at 33mm for N-gauge dual line working.
If you have any questions about how to place aerials, or what aerial sizes and lengths you may need, please contact us before placing an order - we are always willing to discuss your requirements and to answer any queries.

This is the easiest way to install aerials, especially on an existing layout.  However, you need to make sure that the read distance is kept less than 30mm - ideally, a few millimetres less than this will give some overhead.
You should centre the aerial in the middle between the tracks, and use the aerial in-line, that is the 25mm length in the direction of travel.  The easiest way to do this is to drill two small holes (1mm) in the centre of the track, about 30mm apart, and then align the aerial between these holes from the underneath.
With a 9mm(3/8") baseboard, a 3mm track bed, and a normal HO/OO track, the distance to the running rail top will be around 18mm.  With wheels of 12mm diameter, this means that the tag should be placed a couple of millimetres above the line of the axle.

If you have a thick baseboard, you can install the aerial immediately under the sleepers, or under a trackbed such as cork.  If you are doing this on a new layout, then there is little problem .  You can either drill a 8mm hole and feed the aerial wire with housing through it, or a 6mm by 3mm slot hole - done by removing the aerial crimps from the housing, feeding the wires through the baseboard, and then re-assembling the housing.  The aerial can then be stuck to the baseboard, the track laid on top, and then landscaped. 
For existing layouts, you will need to make the hole, remove the landscaping, fit the aerial, lay the track back and then re-landscape.
If you are running N-gauge or below, and want to use the smallest 3mm tags, then you will need to put the aerials under the track. You may put the aerials across the track for N-gauge, but for HO/OO and O gauge, the aerials must be in-line with the track.
For O-gauge, use the 45mm by 30mm aerials - this is an ordering option.
The picture shows a 25mm by 15mm aerial mounted below the sleepers on HO/OO gauge - you could also mount it below the trackbed.

Aerial wires should be taped to the bottom of the baseboard.  They should not be coiled or bundled, and where there is extra length, they should be snaked.  They should be kept 10mm away from other aerials wires.  If you have crossing aerial wires, consider how you could re-route the wires to stop the crossing - it may mean putting the aerials in a different order on a dual or quad reader, or even moving the reader board.
Note that some aerials may have a red stripe on one wire - this is because the aerial wires have been unzipped from a multi-way flat cable - there is absolutely no difference between aerials have a red stripe and those that do not.