I'll start with an internally regulated alternator, as this is the easiest, IMO.
Any of these diagrams can be used on a Type 1 or Type 2 (T1, T2, T3 or T4).
As a rough guide, a Type 1 up to 1973 will have a generator/dynamo. Type 1 from 1974 will have an internally regulated alternator. During 1973 and 4 there were some externally regulated alternators, these are spotted by having a three pin plug in the top like a headlight.
All Type 2 T1s and T2s up to 1973 have generators/dynamos. T2s from 1974, all T3s and all T4s have internally regulated alternators. Like the Beetle, T2s during the cross over period may have externally regulated alternators, so check first.
Looking at the diagram below you'll see the layout required. Pin A on the alternator is the charge lead and has an M5 nut (8mm head) holding down the cable. I've shown the connectors the wrong way around, although on some I've seen Pin B is directly behind Pin A, but it made it easier to draw like this. Pin B is the warning light indicator plug and usually blue.
The relay used can be any value, provided it is at least the same output as the alternator or the maximum input of the battery. For example, a 55 amp alternator and 30 amp battery will only draw 30 amps, but if you had an 85 amp battery, the alternator will only output 55 amps, so a 55 amp relay will be big enough. I personally use a 30 amp relay and have had no problems in 3 years.
The idea is that you only connect the two batteries together when the engine is running and the alternator is outputting a charge current, i.e. the warning light has gone out. To do this, the relay is only switched on when a positive charge comes from Pin B which is used by the dash to turn the light out. When this happens the relay closes the circuit (connects it) between pins 30 and 87 and the second battery can then charge.
Now to wire up a generator/dynamo powered van or car, you'll need to follow the following plans.
Please note that a generator/dynamo can also use the D+ pin on the regulator (or on the generator itself) if this is easier.
If you wish to use an extra battery for starting, neither of these systems will work, instead you'll need to simply have the batteries wired together permanently, but with heavy duty cable. Ask the guy at the auto electric suppliers for jump lead cable, then get an 8mm eye crimped on one end and a regular battery clamp on the other.
You do not need in any of these systems to connect the negative terminals together, simply bolt an earth strap (bought from any auto electric supplier) to the car's/van's metalwork, i.e. floor pan, battery tray or any other suitable part of the car or van.
It might be worth fitting a volt meter along side the auxiliary battery to use as a "condition" meter. If you've got the wire (which will be a few metres on a van) you could add the gauge to the dash, but it is not really needed when driving.