Suggestion number one from me is roll20.net.

Suggestion number two is download a map with a grid, add row and column markers, and have the players state explicitly what their action entails. For example, "I move to square J6, and fire my bow at the goblin at D12."

I recommend MapTool software (its free) if you're using a PC. Its a little finicky to get the hang of but once you get it, it works really well. You can add custom backgrounds, your own tokens, and lots of other cool things. I think it is made for doing live maps but you have the option to export as jpg. You can add a grid, text boxes, coordinates etc. You can also move your tokens around freely.

I use it with Token Stamp 2 to make the tokens and I use either the default backgrounds or have found various free or cheap ones on DMsguild.

I second (or third) the recommendation for Roll20. Easy to use and has quite a lot of features.