If you are unfamiliar with the concept of creating your Asset Allocation, it is basically the blueprints for how you want to invest your money.
For example, it might look like this;
a) I want to have ___% of my investments in equities, and ___% in bonds/cash/savings accounts (should total 100%)
b) Out of my equities, I want
_____% in a domestic large-cap index fund
_____% in a domestic small-cap index fund
_____% in an international large-cap index fund
_____% in emerging markets/small cap
note: these should total 100%
c) out of my bonds/cash savings accounts, I want:
____% in short term bonds/CDs
____% in long-term bonds/CDs
____% in cash/savings accounts.
note: these should total 100%
The above is just an example. You can add REITs, real-estate, gold, foreign currencies, beanie-babies, or exotic cars to your asset allocation.
You can also get much more complex breaking down savings into tax-advantaged (IRA, 401(k)) vs taxable accounts - but I think that's overboard.
The important thing is that you have a plan FIRST. Once you have a plan it's easy to figure out where to put your next slug of cash... (e.g. if you want to be 95% equities /5% bonds/cash and you are 90/10, then you put new investments into equities).
My AA is very simple: I want to be 95/5. In terms of equities, I want 75% to be in my SP500 index fund, and 25% to be in my international index fund. The 5% I hold in my savings account until rates go >2%. It's a very simple but extremely effective AA.
Also - +1 to creating the bogleheads IPS. I didn't go that route, but I didn't know about it at the time.