LH-20 runs as mixed mode. It acts as a GPC (gel permeation chromatography) and also acts (to an extent) as a normal phase column.
Sephadex comes as a dried material and swells when solvent is added. Start with a small amount and add your mobile phase; it will swell, sometimes considerably. The first time I used the stuff, I swelled enough for 10 columns. Let it sit for a couple of hours, decant the fines if desired. I attached a link to the Sephadex instructions at the bottom of this reply; It lists solvents and other useful information.
Sample load is typically 1% to 2% of the volume. Do not let the bed dry out or cracks will form in the column that will reduce resolution. Also, keep the flow rate low. If the flow rate is too high, it causes back pressure which compresses the column, causing more back pressure, more column compression, and the cycle repeats. If using a glass open column, just run the column under gravity flow. If you are running the column on a flash chromatography system (so you can use the UV detector and fraction collector), keep the flow rate down to 1 or 2 mLs per minute unless a large column is being run.