When you're fighting from a distance (in mountains for example, where going forward costs too much time and health), stack the artillery into groups of 10 and distribute all the close combat units across those 10-stacks (preferably add them to the one which you expect to be hit).
When you're attacking or charging on fast terrain, or the enemy close combat units get close, you put the artillery behind with AA, but remember to merge into a single stack (even 10 arty + 10 AA could get destroyed late game). Merging units makes the damage get distributed, so their health and damage efficiency goes down slower and though you receive the same damage at first (just more spread out), as your efficiency stays up more, your enemy loses more health than he would otherwise.

You can hit-and-run by moving and stopping every 30 minutes if you have the time. Also, remember that after you fire, all (!) units in the stack must reload, not only those which actually did damage (eg. if there's 20 arty only 10 does damage due to the army size limit).
Also, if you can't deduce this from the stats:
Artillery counters SP artillery, heavy tanks and tank destroyers.
SP artillery counters all tanks, except LT in some situations, also fair against SPRA.
Rocket artillery counters regular artillery, militia, infantry, commandos, AA/AT...
SPRA counters the same, but regular artillery not so much, and slightly better against faster units (to hit-and-run).
For the strategic aspect, artillery is effective on smaller fronts or chokepoints, and against slow units. Against fast high-attack and low hp units, it loses value as they finish fights faster (win or die) and the artillery has less impact.
Also, you should check the costs of units and work on an army composition (and production ratio) that most effectively uses all your resources (and include economy expenses as well). You could also consider market prices there (eg. leave food over to sell) but the difference between selling and buying is usually too large, so it isn't worth it.

For Railroad Gun, merge them with atleast 1 motorized infantry/Armored car so their viewrange can maximize the usage of railroad gun's attack range

Artillery is great if you know how mechanics in the game works. You've got terrain bonus, then there's that home advantage bonus (useful when in defense of core provinces, early game), then there's stacking other units in it be it for defense against planes or distributing the damage.

Let me put it this way.

Terrain placement is your friend, knowing when to move is your best friend, and microing your artillery stack is your best chance to win fights.

Artillery kinda is like your queen in chess. It can do many great things when you know what youre doing. And if you lose it, its a pretty hard blow.