If you are a seasoned minis painter, please ignore this post, as it is written by an amateur. It is only meant to encourage people who have never threatened their minis with a brush to actually take the leap of faith. I was never good at drawing and crafting, but after watching a few video tutorials, I am now able to produce okay painting results. If you're not a perfectionist, dislike grey plastic, and are quarantined at home, you may consider giving this activity a try.
Materials you will need: I only mention the basics, nothing fancy. Professional paints are not cheap, and if you take this side of the hobby seriously, you may want to invest in quality equipment. As my skills are elementary, I didn't go for expensive materials because they would be wasted. What works for me is:
1) A white spray primer.
2) A cheap set of acrylic paints.
3) A few professional Army Painter paints, just to have some colour variety (especially metallics, which you can't find in the cheap acrylic sets).
4) Paint brushes (one with a very thin tip among them).
5) A plastic palette, and a cup for water.
And that's it. No frills.
You always have to spray the minis with a primer first (white, black or grey are the most common base colours), to help the paint stick on the mini. Do this outside, in your balcony or yard, and use a mask from your covid-19 supplies, to avoid inhaling the toxic fumes. The spray dries in seconds.
Now, painting. The most important decision is the colour scheme. Imagining the final outcome is not so easy, and I sometimes look at illustrations from the game the mini belongs to, to get ideas. This doesn't mean one can't get creative, of course. Especially when a game has, let's say, a lot of identical zombies, having them all wear blue pants and brown shirts will make you bored out of your skull. Try to differentiate the models a little, and why not, disrespect all conventions. Most people who have painted the menhirs in Tainted Grail have given them natural hues, mostly stone-grey. But not this BGG member. Do I like it? Well, no. But whether it is a fitting colour combination matters little. What does matter is that she most likely had fun painting.
A painted mini looks good if the details of the sculpt are highlighted. There are two basic ways to achieve this: dry-brushing, and washing with an ink paint. Dry-brushing is literally what the name says: dip a dry, not wet, brush in a little paint, wipe most of the paint on a paper towel, then brush your mini. This technique really brings the details out. An ink is a very diluted colour, usually black or blue, that you can apply to the mini after you paint it. It sits in all the nooks and crannies, and livens up faces, draped cloths, scaly skins etc.
The good thing is that, if you are not satisfied with the end result, you can paint over it again. Sure, some minis are more difficult to paint than others, and especially very small ones can be a pain. When you place your painted minis on the board, however, you will be congratulating yourself. They may even motivate you to play a game that had been sitting on the shelf for too long. I painted my Nemo's War submarine yesterday. Haven't played the game in ages. Now I'm eager to see my shiny vessel traverse the oceans.
This video tutorial explains the basics quite well.