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@ntaw has the basics there.

I would also add that depending on the model and the join you may need to use larger or thicker gauge wire than a paperclip. You may also need to reinforce the join with modeling putty or greenstuff.

A few things to keep in mind. You'll want to have some sort of a vice handy and you'll also need a protective wrapper. In some cases it will be very difficult to hold the piece in your hand when drilling, if it is awkwardly shaped or if it is very small. Then it can be helpful to wrap it in a protective layer before clamping it in a vice. I typically use the card stock from the back of a blister pack to prevent the jaws of the vice from deforming the pewter.

It's always a balance between how much force you use to hold it still and avoiding damaging the model.

I don't have a table vice so I usually use vice-grip pliers, which also have a knurled gripping surface which makes a protective layer even more important.

 

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The drill is actually called a "pin vise." That links to an amazon page with an example.

Here's another example I like better because it has a swivel head which allows you to use a lot more force without hurting your hands.

 

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Another great tip is to paint one entire side of the join and press them together. If you need to find a door for the hole OR if you need to check how smoothly the halves come together.

Using paint this way instantly colors the raised areas which need to be sanded down.

 

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Little while ago I got into a Battletech kick so purchased a bunch of models. All metal. Decided that this time (unlike when I was 10) that I was going to pin them. Unlike a plastic model where I made the pin "straight", meaning that I drilled straight into the arm and straight into the body, I decided to try a new method for the metal. For the arm I did an angle up (looked like this: \) and on the body I did an angle down (looked like this:\). That way, because gravity is pulling the arm down, it's pulling it constantly down onto the pin and constantly pushing the pin down in the body cavity. Has worked well for me. Did some playing with them, painting, and even a packing and move, and no broken pieces. Just my two cents and an alternate way of doing it.

Great point. I've done similar things. Definitely consider how the model will be handled and how gravity will affect it.

With some larger models it's sometimes necessary to use more than 1 pin as well.

I had to do that in the original chaos dragon model, Baudros. If you look in the lower left of the image there is a join between the lower body and upper body with very little support.

Here's the full model.

As you can see there's a LOT of pewter above that join.

 

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@morfangdakka wow, that's a pretty serious model for a "pewter beginner." I was young when I made mine, but I had already made a bunch of smaller metal models.