Stilly, yes not all plant foods have all the right amino acids in the right proportions for the body to synthesise protein from. Grains have more of some than others, while the balance will be different in seeds & nuts, and pulses (lentils & beans).

The traditional thinking was that you needed to 'combine' two of three of these different food groups (nuts with grains / nuts with beans / beans with grains / grains with nuts) at any one meal to be sure of getting the full complement of amino acids needed.

However modern nutrition science thinking is a lot more relaxed about specifically combining these three food groups in meals, as the body stores amino acids for later synthesis of protein - they don't get flushed away like water soluble vitamins for example. So long as your diet is balanced and varied, and mostly based on 'natural' foods or wholefoods, you should be OK.

I don't personally tend to think about combining these food groups, on a whole-food based diet it tends to happen for me anyway - as a byproduct of just eating certain foods I like. That may be because - by either happy coincidence or experience based pragmatism - a lot of traditional peasant foods tend to combine those foods anyway.

The staple foods of the rural poor in any culture tends (or has tended - this changes significantly with increased economic prosperity) to be predominantly plant based, so you will find a lot of suitable traditional recipes from around the world that fit into a veg diet very well. I make use of them myself.

Here's an example of how the kinds of foods I regularly eat which accidentally combine the above food groups: Hummus and pita breads (grain and bean), hopping john (grain and bean), nut roast (nuts with grain) bean burgers (beans with grain), minestrone soup (grain and bean), lentil soup with bread (bean and grain), etc.

So it's pretty easy to do. Though it's worth keeping a note of what your food habits are so that you know where you may be lacking, or where you're going wrong. As said below, a food tracker is a useful tool, at least initially until you know where you are with a new dietary routine.