There are basically two ways to figure 401k employee/employer contribution limits, one for proprietorship/llc (where you don't pay yourself a salary) and then one for something like a S Corp (where you do).
In your case the calculation is a little more complicated as far as the limit goes for the employer side (here is a calculator I like that shows both), but once you know what your limits are actually making the contributions for both sides are simple.
I use Vanguard Small Business for mine and every quarter I figure what I can contribute and enter the transaction and Vanguard deducts it straight from my business checking account for both employer and employee sides. If you were paying yourself a salary (e.g. if you elected to file as an S COrp) then you;d have to factor this into your W-2 to yourself, etc. But as a straight pass thru like a sole prop or LLC its just as simple as buying the investments within your solo 401k (designating which are employee and which are employer).
http://www.bcmadvisors.com/solo-401k-calculator